Air Sampling--Statistical Analysis and Relevance

This section contains theoretical and real-world discussions about statistical analysis. It gives special emphasis to situations where knowledge of statistical relevance is a prerequisite for sampling adequacy. It also illustrates the difference between log-normal and normal distribution and parametric monitoring data.

1. DEFINITIONS

In statistical analysis and relevance, certain standard definitions are used. The following standard definitions and examples illustrate the basic concepts.

• Accuracy: Agreement of the measured value (i.e., empirical value) and the "true value'' (i.e., accepted reference value) of the sample given valid sampling techniques, proper sample preparation, and reliable and accurate instrumentation and/or other procedures.

Accuracy is often estimated by adding (or "spiking'') known amounts of the target parameters. For asbestos quality assurance (QA) sampling, accuracy is evaluated by comparing analyses of duplicate samples that have been evaluated in proficiency in analytical testing (PAT) round robins (for air samples) or NIST National Voluntary Laboratory Accreditation Program (NVLAP) proficiency testing (for bulk and transmission electron microscopy [TEM]).

Accuracy is a measure of the bias of the method and may be expressed as the difference between two values, a ratio, or the percentage difference.

• Analysis: Combination of sample preparation and evaluation.

• Audit: Systematic determination of the function or activity quality.

• Bias: Systemic error either inherent in the method or caused by measurement system artifacts or idiosyncrasies.

• Blind sample: Presented to the laboratory as indistinguishable from field samples (syn: performance audit samples). All field blanks are to be presented to the laboratory as blind samples for asbestos air samples.

• Calibration: Comparative procedure in which singular measurements are evaluated against an accepted group of measurements. The evaluation may be against a primary, intermediate, or secondary standard.

• Calibration curve: Range over which measurement can take place (syn: standard curve, multipoint calibration).

• Calibration standard: Instruments or other measurement techniques used to evaluate the measurement system. Ideally, these standards don’t directly incorporate or use the target parameters to be measured.

• Chain-of-custody: Defined sample custody procedures that must be followed to document the transition from field collection to subsequent transfer sites (common carriers, laboratories, storage facilities, etc.).

• Check standard: Used to verify that the initial standard or calibration curve remains in effect. It ideally incorporates standard materials (syn: daily standard, calibration check or standard, reference standard, control standard, single point response factor, single point drift check).

• Comparability: Confidence with which one set of empirical data can be com pared to another.

• Completeness: Amount of valid data obtained from a measurement system compared to the amount expected.

• Data quality: Totality of data parameters that identify ability to satisfy or represent a given condition; includes accuracy, precision, representativeness, and comparability.

• Data reduction: Using standard curves to interpret raw data.

• Data validation: Review process that compares a body of data against a set of criteria to provide data adequacy assurance given the data's intended use; includes data editing, screening, checking, auditing, verification, certification, and review.

• Detection limit: Minimum target parameter quantity that can be identified, i.e., distinguished from background or "zero'' signal.

• Double-blind sample: When neither the composition nor identification of the sample is known to the analyst.

• Duplicate sample (field or laboratory): Sample divided into two portions, with both portions carried through the sample preparation process at the same time.

For asbestos air samples, field duplicates are air samples collected at the same time as the compliance air samples, and lab duplicates are portions of one filter that are fixed and analyzed separately.

• Environmentally related measurements: Field or laboratory investigations that generate data involving chemical, physical, or biological parameters characteristic of the environment.

• Field blanks: Generated at the time of sampling, field blanks provide a check on contamination, starting with the sampling process and proceeding through the full analysis scheme. For asbestos fiber concentration sampling, field blanks are filter cassettes transported to the site and exposed to ambient conditions. The filter caps are removed from the filter cassettes; however, a vacuum air pump is not used to pull air across the filter cassettes. Thus, the cassettes are exposed to the environmental airstream of the surrounding environment outside the asbestos control area.

• Good laboratory practices: Performing a basic laboratory operation or activity so as not to influence data generation quality.

• Instrument blank: Used to obtain information on instrument aberration absence/presence. The measurement instrument is presented with materials normally within the instrument and cycled through the measurement sequence.

The resulting signal is then defined as the baseline instrument signal level.

• Internal standard: A nontarget parameter added to samples just prior to measurement to monitor variation in sample introduction and stability and to normalize data for quantitation purposes. Internal standards are not usually used in bulk (phase light microscopy [PLM]) or phase contrast microscopy (PCM); however, these standards may be applicable to TEM protocol.

• Laboratory blank: Prepared in the laboratory after receipt of samples from the field. These blanks are prepared using a material assumed not to contain the tar get parameter. Lab blanks for asbestos sampling are filter membranes obtained from filter cassettes that have been retained in the laboratory without removal of the filter cassette caps. The lab blank is a check on all the chemicals and reagents used in the method as well as the influence of the general laboratory environment (syn: analytical blank, system blank, method blank, process blank).

• Measurement: Creating quantitative data from a prepared sample.

• Method check sample: Prepared in the laboratory by spiking a clean reference matrix with known quantities of the target parameters. For asbestos air-sampling analysis, method check samples are previously prepared filters evaluated by separate analysts within the same laboratory. These duplicate analyses are defined in the National Institute of Occupational Safety & Health (NIOSH) 7400 method as quality assurance, and the acceptable statistical parameters are outlined therein.

• Method detection limit: Minimum quantity that a method (i.e., both sample preparation and target parameter measurement steps) can be expected to distinguish from background or "zero'' signal. This limit takes into account losses during preparation and measurement and instrument sensitivity that may contribute to qualification or quantification of results. This limit does not apply to physical parameters (i.e., density, temperature).

• Performance evaluation (PE) sample: Sample with known "true'' values that is presented to the laboratory as a "performance evaluation sample.'' These samples are biased by the analyst's knowledge of the intent of the sample. For asbestos air-sampling analysis, "true'' value samples may be defined as the PAT samples with their inclusive statistical ranges.

• Precision: Measure of the reproducibility of a set of results obtained under similar conditions. Precision is determined by multiplicate analysis of samples, duplicates, replicates, or splits. Standard deviation is used as a measure of precision.

• Procedure: Systematic instructions and operations for using a method of sampling or measurement.

• Proficiency sample: Samples for which known composition values are available for accuracy comparisons. The composition values may be qualitative, quantitative, or statistical ranges of acceptable qualitative/quantitative results.

• Quality assurance: An orderly assemblage of management policies, objectives, principles, and general procedures by which a laboratory outlines the methods used to produce quality data. QA is an intra-laboratory function. Note: The NIOSH 7400 method defines QA in terms of both intra-laboratory and inter-laboratory methods and/or sequencing. However, for the purposes of specified QA/quality control (QC) documents, inter-laboratory methods are defined as QC.

• Quality control: Routine application of procedures used to develop prescribed performance standards in the monitoring and measurement of standards. QC is an inter-laboratory function.

• QC samples: Analyzed concurrently with field samples to insure that analytical systems are operating properly, i.e., in control. These samples provide an estimate of the precision and accuracy of the sampling and analysis system. QC samples for asbestos sampling are sent between laboratories for interlaboratory comparisons of methodology and analytical proficiency.

• Quality of method: Degree to which the method functions free of systemic error, bias, and random error.

• Quantitation limits: Maximum and minimum levels or quantities of a reliably quantified target parameter. These limits are bounded by the standard curve limits and are generally related to standard curve data.

• Reagent blank: Used to identify contamination sources. These blanks incorporate specific reagents during sample preparation to identify lab blank contaminate sources (syn: dilution blank).

• Recovery/percent recovery: Generally used to report accuracy based on the measurement of target parameters, comparison of these concentrations, and correlating these measurements to the predicted amounts. Recovery in asbestos air sampling is the statistical percentage differential observed during accuracy evaluation (i.e., percentage difference in fiber concentrations).

• Replicate samples (field or laboratory): A sample is divided into two portions and is processed as two completely separate and nonparallel samples, i.e., pre pared and analyzed at different times or by different people. Field replicates in asbestos air sampling are defined as filters that are obtained from two separate filter cassettes drawn separately or from a y-juncture. These are then transported, fixed, and analyzed separately. Lab replicates are taken from a singular filter, which is sectioned, fixed, and analyzed separately.

• Representativeness: Degree to which data accurately and precisely represent a parameter variation characteristic at a sampling point and portray an environ mental condition.

• Sample custody: Verification and documentation procedure for the transfer of samples from the field to the laboratory, within the laboratory, and to the final storage or disposal destination.

• Sample Operation Procedure (SOP): Procedure adopted for repetitive use when performing a specific measurement or sampling operation.

• Sample preparation: Transformation of the sample into appropriate forms for transfer and/or measurement.

• Sampling: Removal of a process stream representative portion or a portion of a larger quantity of material for subsequent evaluation.

• Sensitivity: An instrument's detection limit given the minimum quantity of a tar get parameter that can be consistently identified, i.e., distinguished from back ground or "zero'' signal by the instrument; ideally established using the materials that are used for standardization.

• Split samples (field or laboratory): A sample, divided into aliquots, that is sent to a different laboratory for preparation and measurement. These split samples may be replicates or duplicates that are then defined as splits when sent to another laboratory for QC analysis. For asbestos air samples this shipment involves either the shipping of capped filter cassettes (field split) or the shipping of fixed slides (lab split).

• Standard materials: Materials, such as mixtures of the target parameters at known concentration and purity, used to carry out standardizations. For asbestos air sampling these materials may be PAT samples.

• Standardization: Establishing a quantitative relationship between known target parameters input and instrument readout.

• Target parameters: Entity for which qualitative or quantitative information is desired.

• Trip blanks: Essentially field blanks that don’t have the caps removed. These blanks provide insight into the contamination generated as a result of the shipping process. These blanks are generally not required for asbestos sample shipment.

2. EXAMPLE--OUTLINE OF BULK SAMPLING QA/QC PROCEDURE

Bulk sample analysis procedures are defined in the NIOSH method for PLM and the NIOSH 7402 method for TEM. Because bulk sample analysis is done by an independent laboratory off-site, this QA/QC document won’t address bulk sampling as a field verified procedure. The NVLAP is currently used to ascertain laboratory effectiveness. The contractor must provide proof that NVLAP certification is current for the laboratory designated to receive both the initial bulk samples and the 10% bulk sample duplicates.

The optical properties and ID of fibers are as follows:

• Determine 13 specific items for asbestos.

• ID other fibers with some optical data.

• ID matrix components.

In addition record

• Special procedures or solvents

• Sampling of layers

• Deviations from EPA test procedure The 10% bulk sample duplicates are provided by physically dividing 10% of the samples collected. This division should occur concurrently with the collection of field samples.

The rationale for splitting the samples at a time and location removed from the field collection site must be provided.

All field collection procedures, sample labeling, and transport and disposal procedures must be addressed in the QA/QC document. Provide the rationale for the classification and remediation of errors. The following is a sample of error classification:

Major Error

• False positive

• False negative (asbestos actually _1%)

• Incorrect asbestos type classification

• Analysis quantification in error by 25% Minor Error

• Incorrect ID of tremolite or actinolite as another type of asbestos

• False negative (asbestos actually _1% or trace)

• Analysis quantification in error by 15%

• Incomplete lab data sheets (repeated omissions may equal major error) Corrective Steps-Major Errors

• Take immediate action.

• Review documents for transcription errors.

• Review sample, especially matrix description.

• Reanalyze sample; submit to other labs for analysis.

• Check environment for contamination and out of calibration equipment.

• For misidentification of asbestos species review literature and reference samples.

Corrective Steps-Minor Errors

• One to two weeks or after monthly summaries

• Steps 1 through 4 above.

• Review to determine if error is systematic estimation bias, then retrain on estimation techniques and/or sample preparation techniques.

• Completed data sheets are required; frequent omissions should be considered a major error.

• Review sampling and stereomicroscope sample preparation procedures.

• To recalibrate estimation techniques, reanalyze known samples; review literature on estimation training.

• Review; retrain on reference samples, especially in problem matrix mixtures.

Review specific problems like tremolite or crocidolite samples.

• Review lab's special sample preparations for Vinyl Asbestos Tile (VAT) or tar matrix materials. Practice on known materials including blanks.

3 EXAMPLE-OUTLINE OF THE NIOSH 7400 QA PROCEDURE

3.1 Precision: Laboratory Uses a Precision of 0.45

Current guide specifications give a precision of 0.45 as acceptable in calculation of the 95% upper confidence level (UCL). Using a precision of 0.45 implies that the standard deviation divided by the arithmetic mean gives a value of 0.45. This ratio is variously called the coefficient of variation (CV) or the SR in the NIOSH 7400 method. With the 90% confidence interval of mean count, which includes a subjective component of 0.45 plus the Poisson component, 0.45 precision implies that reproducibility of results is questionable. Thus, the use of the 0.45 value in the calculation of the UCL must be clearly identified. For compliance purposes the following equation is acceptable:

Measured _ (air quality) (0.45) (1.645) concentration (standard) The QA procedures given in the NIOSH 7400 method must be referenced in a discussion of the 0.45 precision value used. Detailed outlines of the intralaboratory procedures are not necessary. Proof of acceptable PAT participation (i.e., judged proficient for the tar get parameter in four successive round robins) as administered by the American Board of Industrial Hygiene (ABIH) must be provided.

3.2 Precision: Laboratory Uses a Precision SR that is Better Than 0.45

When a precision better than 0.45 is suggested, an outline of the NIOSH 7400 QA procedures used and a current CV curve must be provided in addition to the proof of accept able PAT participation. The outline must include the following:

• Document the laboratory's precision for each counter for replicate fiber counts by using this procedure.

• Maintain as part of the QA program a set of reference slides to be used daily.

These slides should consist of filter preparations including a range of loading and background dust levels from a variety of sources including both field and PAT samples.

-Have the QA officer maintain custody of the reference slides and supply each counter with a minimum of one reference slide per workday. Change the labels on the reference slides periodically so that the counter does not become familiar with the samples.

-From blind repeat counts on the reference slides, estimate the laboratory intra and intercounter SR.

-Determine separate values of relative standard deviation for each sample matrix analyzed in each of the following ranges. Maintain control charts for each of these data files.

• 15 to 20 fibers in 100 graticule fields

• _20 to 50 fibers in 100 graticule fields

• _50 to 100 fibers in 100 graticule fields

• 100 fibers in less than 100 graticule fields

Note: Certain sample matrices (e.g., asbestos cement) have been shown to give poor precision. Prepare and count field blanks along with the field samples. Report counts on each field blank.

Note 1: The identity of blank filters should be unknown to the counter until all counts have been completed.

Note 2: If a field blank yields greater than 7 fibers per graticule fields, report possible contamination of the samples.

• Perform blind recounts by the same counter on 10% of filters counted (slides relabeled by a person other than the counter). Use the following test to determine whether a pair of counts by the same counter on the same filter should be rejected because of possible bias:

• Discard the sample if the absolute value of the difference between the square roots of the two counts (in fiber/mm^2) exceeds 2.8 (x) 2 _ 8 _ SR, where x _ the average of the square roots of the two fiber counts (in fiber/mm^2) and SR _ one half the intracounter relative standard deviation for the appropriate count range (in fibers).

Note 1: Fiber counting is the measurement of randomly placed fibers that may be described by a Poisson distribution; therefore, a square root transformation of the fiber count data will result in approximately normally distributed data.

Note 2: If a pair of counts is rejected by this test, recount the remaining samples in the set and test the new counts against the first counts. Discard all rejected paired counts. It’s not necessary to use this statistic on blank counts.

The analyst is a critical part of this analytical procedure. Care must be taken to provide a non-stressful and comfortable environment for fiber counting. An ergonomically designed chair should be used. With the microscope eyepiece situated at a comfortable height for viewing. External lighting should be set at an intensity level similar to the illumination level in the microscope to reduce eye fatigue. Counters should take 10-20 min breaks from the microscope every 1 to 2 hours to limit fatigue. During these breaks both eye and upper back/neck exercises should be performed to relieve strain.

Calculation of compliance uses the same equation with the substitution of a different value for 0.45. Use of this different value requires prior approval. All filters selected for the 10% recount should be randomly selected, rather than selecting every tenth filter.

3.3 Records to Be Kept in a QA/QC System

Records include:

• Sample logbook

• Chain-of-custody record

3.4 Field Monitoring Procedures-Air Sample

Collection of 10% duplicates as provided by specifications are not to be confused with the 10% duplication used for QA in the NIOSH 7400 method. The 10% duplicates provided by specifications are for QC interlaboratory determination of reliability. These duplicates are collected in the field using simultaneous collection devices or analysis preparatory techniques. Running two pumps on one individual is feasible for personal air sampling.

Running a Y shunt to two separate collection cassettes may be the technique used for area monitoring simultaneous collection. This method may also be feasible for personnel monitoring.

When neither type of collection alternative is feasible, duplication of prepared slides through duplicate filter media mounting is acceptable. The rationale and techniques for all prospective alternatives must be provided in the QA/QC document.

Caution: Field blanks must indeed be field blanks, not randomly prepared laboratory blanks. The purpose of field blanks is to access ambient air particulates hypothetically unassociated with abatement activities, but existing in the same general geographic location. Thus, field blanks must be filter-loaded cassettes that are open faced and stored out side the asbestos control area in an associated support zone. Occasionally, hygienists carry the field blanks in their pockets. This practice, if employed, must be documented, with associated persuasive rationale.

3.5 Calibration

All calibration techniques and schedules must be provided. Calibration of air-sampling devices may be accomplished using a primary standard (such as a bubble burette) or using a precision rotameter calibrated against a primary standard. The bubble gauge installed on the front vertical face of personal air-sampling pumps is not a reliable or precise calibration tool. Calibration must be done using dial settings versus primary standard timings.

Rotameters offer an advantage in that their portability makes it possible to calibrate cassettes with the associated vacuum pump-generated airstream (or train) at the worksite.

Precision rotameters, while preferred, can be supplemented with standard rotameters.

Techniques for rotameter use and calibration must be sequentially and clearly defined. The use of long-gauge range, 0-20 l/min, rotameters in the measure of personal air-sampling pumps with expected 2.5 l/min is prohibited due to lack of precision. Short-gauge range,

0-5 l/min rotameters, are allowed for use in the calibration of personal air sampling pumps. Example calibration curves and associated calculations must be provided.

Calibration of Phase Contrast Microscopy (PCM) microscopes used on-site must be completely defined. All NVLAP and American Conference Governmental Industry Hygienists (ACGIH) calibration procedures are enforceable for on-site activities. While not generally defined as a calibration technique, cleanliness and relative stability of the PCM microscopic location must be addressed. Sample calibration checklists used daily and weekly must be provided.

3.6 Negative Air Pressure

When negative air pressure is used in gross containment areas, monitoring criteria must be provided. Specifications should require continued real-time instrument monitoring independent of the HEPA vacuum system monitors. Monitors must be located outside the containment area and removed from the effluent HEPA vacuum airstream. Appropriate monitoring checklists and sample direct readout tapes must be provided. The readout tapes and associated calculations, if needed, can either be generically presented or be samples from previous monitoring efforts.

3.7 Compressor

In the event that compressed air is used on-site, certification of Grade D breathing air must be provided. If a filter bank is used in conjunction with an oil-lubricated compressor, monitoring of the filter bank is required. This monitoring includes carbon monoxide, temperature, oil breakthrough, and air pressure criteria. In addition to audible alarms and escape air, visual monitoring of the compressor filter bank status is necessary. Provide sample monitoring check sheets. Certain specifications call for the use of colorimetric tubes to certify continued Grade D breathing air supply. Provide sample monitoring check sheets that clearly indicate sampling intervals.

3.8 Recordkeeping and Sample Storage

Recordkeeping priorities and samples of format used must be provided. Records include documentation of air sampling and air sample analysis, bulk sampling and bulk sample analysis, and negative air pressure maintenance. Personnel records include résumés, certifications, medical surveillance, and training. Environmental records detail work sequencing and ambient conditions. All specified documents must be accessible and maintained as per specifications. Sample storage and accessibility must be discussed.

The following equipment and documentation lists are examples of those that should be appraised for asbestos bulk sampling. Other laboratory protocols will require similar lists.

Laboratory Procedures

• Logbook

• Calibration of refractive index oils

• Daily microscope alignment

• Daily microscope calibration check

• Daily microscope contamination check

• Equipment maintenance

• Equipment calibration

• Personnel records, including hierarchy, training, certification, and job descriptions

• Monthly records of each analyst

• QA and QC results for their work

• Proficiency results (PAT)

• Precision and accuracy ratings, including explanation of rating protocols QA Logbook

• Samples

• Results

• Discrepancies

• Analysis repeats (minimum 10%)

• Intralaboratory analysis of proficiency samples

• Intralaboratory analysis of duplicates and replicates

• Blank analysis

• Summary of results from each analyst

• Summary of results for the laboratory QC Logbook

• Deficiency corrections

• Samples

• Results

• Discrepancies

• Frequency of duplicate/replicate analysis per total samples

• Interlaboratory analysis of proficiency samples

• Timing of QC analysis

• Same day, next shift, next day

• Monthly proficiency samples, WULAP samples in-house or past EPA asbestos bulk sample analysis, QA program samples, blanks, and contamination samples

Interlaboratory Analyses (summary of results for the laboratory)

• Outliers

• Interlaboratory analysis schedules

• Time, including expected turnaround time

• Labs participating

• Contamination testing and control logbook

• Lab data sheet/notebook

• Analysis report sheet

• QA manual revision documentation

• Training procedures for staff

• Analysis error correction correspondence

4. SAMPLING AND ANALYTICAL ERRORS

When an employee's personal exposure or the area exposure is sampled and the results analyzed, the measured exposure will rarely be the same as the true exposure. This variation is due to sampling and analytical errors or SAEs. The total error depends on the combined effects of the contributing errors inherent in sampling, analysis, and pump flow.

Error factors determined by statistical methods shall be incorporated into the sample results to obtain:

• The lowest value that the true exposure could be (with a given degree of confidence)

• The highest value the true exposure could be (also with some degree of confidence) The lower value is called the lower confidence limit (LCL), and the upper value is the UCL.

These confidence limits are one sided, since the only concern is with being confident that the true exposure is on one side of the Permissible Exposure Limit (PEL).

4.1 Determining SAEs

SAEs that provide a 95% confidence limit have been developed and are listed on the OSHA-91B report form (most current SAEs). If there is no SAE listed in the OSHA-91B for a specific substance, call the laboratory. If using detector tubes or direct-reading instruments, use the SAEs provided by the manufacturer.

4.2 Environmental Variables

Environmental variables generally far exceed SAE. Samples taken on a given day are used by OSHA to determine compliance with PELs. However, where samples are taken over a period of time (as is the practice of some employers), the industrial hygienist should review the long-term pattern and compare it with the results. When OSHA's samples fit the long-term pattern, it helps to support the compliance determination. When OSHA's results differ substantially from the historical pattern, the industrial hygienist should investigate the cause of this difference and perhaps conduct additional sampling.

4.3 Confidence Limits

One-sided confidence limits can be used by OSHA to classify the measured exposure into the following categories.

95% Confident That the Employer Is in Compliance

• Measured exposure results don’t exceed the PEL.

• UCL of that exposure does not exceed the PEL.

95% Confident That the Employer Is NOT in Compliance

• Measured exposure results do exceed the PEL.

• LCL of that exposure does exceed the PEL.

95% Confident That the Employer Is in Compliance _ Possible Overexposure

• Measured exposure results don’t exceed the PEL.

• UCL of that exposure does exceed the PEL.

NOT 95% Confident That the Employer Is NOT in Compliance _ Possible Overexposure

• Measured exposure results do exceed the PEL.

• LCL of that exposure does not exceed the PEL.

A violation is not established if the measured exposure is in the "possible over exposure'' region. It should be noted that the closer the LCL comes to exceeding the PEL, the more probable it becomes that the employer is in noncompliance.

If measured results are in this region, the industrial hygienist should consider further sampling, taking into consideration the seriousness of the hazard, pending citations, and how close the LCL is to exceeding the PEL.

If further sampling is not conducted, or if additional measured exposures still fall into the "possible overexposure'' region, the industrial hygienist should carefully explain to the employer and employee that the exposed employee(s) may be overexposed, but that there were insufficient data to document noncompliance. The employer should be encouraged to voluntarily reduce the exposure and/or to conduct further sampling to assure that exposures are not in excess of the standard.

5. SAMPLING METHODS

The LCL and UCL are calculated differently depending on the type of sampling method used. Sampling methods can be classified into one of three categories:

5.1 Full-Period, Continuous Single Sampling

Full-period, continuous single sampling is defined as sampling over the entire sample period with only one sample. The sampling may be for a full-shift sample or for a short period ceiling determination.

5.2 Full-Period, Consecutive Sampling

Full-period, consecutive sampling is defined as sampling using multiple consecutive samples of equal or unequal time duration that, if combined, equal the total duration of the sample period. An example would be taking four 2-hour charcoal tube samples.

There are several advantages to this type of sampling. If a single sample is lost during the sampling period due to pump failure, gross contamination, etc., at least some data will have been collected to evaluate the exposure. The use of multiple samples will result in slightly lower SAE. The collection of several samples leads to conclusions concerning the manner in which differing segments of the workday affect overall exposure.

5.3 Grab Sampling

Grab sampling is defined as collecting a number of short-term samples at various times during the sample period that, when combined, provide an estimate of exposure over the total period. Common examples include the use of detector tubes or direct-reading instrumentation (with intermittent readings).

6. CALCULATIONS

If the initial and final calibration flow rates are different, a volume calculated using the highest flow rate should be reported to the laboratory. If compliance is not established using the lowest flow rate, further sampling should be considered.

Sampling is generally conducted at approximately the same temperature and barometric pressure as calibration, in which case no correction for temperature and pressure is required, and the sample volume reported to the laboratory is the volume actually measured. Where sampling is conducted at a substantially different temperature or pressure than calibration, an adjustment to the measured air volume may be required depending on sampling pump used, in order to obtain the actual air volume sampled. The actual volume of air sampled at the sampling site is reported and used in all calculations.

For particulates the laboratory reports milligrams per cubic meter of contaminant using the actual volume of air collected at the sampling site. This value can be compared directly to OSHA Toxic and Hazardous Substances Standards (e.g., 29 CFR 1910.1000).

The laboratory normally does not measure concentrations of gases and vapors directly in parts per million. Rather, most analytical techniques determine the total weight of contaminant in a collection medium. Using the air volume provided by the industrial hygienist, the lab calculates the concentration in milligrams per cubic meter and converts this to parts per million at 25°C and 760 mmHg. This result is to be compared with the PEL with out adjustment for temperature and pressure at the sampling site.

ppm(NTP) _ mg/m^3 (24.45)/(Mwt) where

• 24.45 _ molar volume at 25°C (298 K) and 760 mmHg

• Mwt _ molecular weight

• NTP _ normal temperature and pressure at 25°C and 760 mmHg

If it’s necessary to know the actual concentration in parts per million at the sampling site, it can be derived from the laboratory results reported by using the following equation:

ppm(PT) _ ppm(NTP) (760)/(P) (T)/(298) where

•P _ sampling site pressure (mmHg)

•T _ sampling site temperature (K)

• 298 dgr. temperature in K Since ppm(NTP) _ mg/m^3 (24.45)/(Mwt) ppm(PT) _ mg/m^3 _ 24.45/Mwt _ 760/P _ T/298

Note: When a laboratory result is reported as milligrams per cubic meter contaminant, concentrations expressed as parts per million (PT) cannot be compared directly to the standards table without converting to NTP.

Note: Barometric pressure can be obtained by calling the local weather station or air port and requesting the unadjusted barometric pressure. If these sources are not available, then a rule of thumb is for every 1000 ft increase in elevation, the barometric pressure decreases by 1 in. Hg.

6.1 Calculation Method for a Full-Period, Continuous Single

Sample

Obtain the full-period sampling result (value X), the PEL, and the SAE. The SAE can be obtained from the OSHA Chemical Information Manual. Divide X by the PEL to determine Y, the standardized concentration, that is, Y _ X/PEL.

Compute the UCL (95%) as follows: UCL (95%) _ Y _ SAE Compute the LCL (95%) as follows: LCL(95%) _ Y - SAE Classify the exposure according to the following classification system:

• If the UCL _ 1.0, a violation does not exist.

• If the LCL _ 1.0 and the UCL _ 1, classify as possible overexposure.

• If the LCL _ 1.0, a violation exists.

6.2 Sample Calculation for a Full-Period, Continuous Single

Sample

A single fiberglass filter and personal pump were used to sample for carbaryl for a 7-h period. The industrial hygienist was able to document that the exposure during the remaining unsampled 0.5 h of the 8-h shift would equal the exposure measured during the 7-h period. The laboratory reported 6.07 mg/m^3. The SAE for this method is 0.23. The PEL is 5.0 mg/m^3.

Step 1. Calculate the standardized concentration.

Y _ 6.07/5.0 _ 1.21

Step 2. Calculate confidence limits.

LCL _ 1.21 - 0.23 _ 0.98 Since the LCL does not exceed 1.0, noncompliance is not established. The UCL is then calculated: UCL _ 1.21 _ 0.23 _ 1.44

Step 3. Classify the exposure.

Since the LCL _ 1.0 and the UCL _ 1.0, classify as possible overexposure.

6.3 Calculation Method for a Full-Period Consecutive Sampling

The use of multiple consecutive samples will result in slightly lower SAEs than the use of one continuous sample because the inherent errors tend to partially cancel each other.

The mathematical calculations, however, are somewhat more complicated. If preferred, the industrial hygienist may first determine if compliance or noncompliance can be established using the calculation method noted for a full-period, continuous, single-sample measurement. If results fall into the "possible overexposure'' region using this method, a more exact calculation should be performed as follows.

Compile X(1), X(2) ..., X(n), and the n consecutive concentrations on one workshift.

Compile their time durations, T(1), T(2), ..., T(n).

Compile the SAE.

Compute the TWA exposure.

Divide the TWA exposure by the PEL to find Y, the standardized average (TWA/PEL).

Compute the UCL (95%) as follows: UCL (95%) _ Y _ SAE (Equation E).

Compute the LCL (95%) as follows: LCL (95%) _ Y _ SAE (Equation F).

Classify the exposure according to the following classification system:

• If the UCL _ 1.0, a violation does not exist.

• If the LCL _ 1.0, and the UCL _ 1, classify as possible overexposure.

• If the LCL _ 1.0, a violation exists.

When the LCL _ 1.0 and the UCL _ 1.0, the results are in the "possible overexposure: region, and the industrial hygienist must analyze the data using the more exact calculation for full-period consecutive sampling, as follows:

6.4 Sample Calculation for Full-Period Consecutive Sampling

Two consecutive samples were taken for carbaryl instead of one continuous sample, and the following results were obtained:

The SAE for carbaryl is 0.23.

Step 1. Calculate the UCL and the LCL from the sampling and analytical results:

Step 2. Since the LCL _ 1.0 and the UCL _ 1.0, the results are in the possible over exposure region, and the industrial hygienist must analyze the data using a more exact calculation for full-period consecutive sampling. If the LCL_1.0, a violation is established.

7. GRAB SAMPLING

If a series of grab samples (e.g., detector tubes) is used to determine compliance with either an 8-h TWA limit or a ceiling limit, consult with an industrial hygienist (ARA) regarding sampling strategy and the necessary statistical treatment of the results obtained.

8. SAES--EXPOSURE TO CHEMICAL MIXTURES

Often an employee is simultaneously exposed to a variety of chemical substances in the workplace. Synergistic toxic effects on a target organ are common for such exposures in many construction and manufacturing processes. This type of exposure can also occur when impurities are present in single chemical operations. New PELs for mixtures, such as the recent welding fume standard (5 mg/m^3 ), addresses the complex problem of synergistic exposures and their health effects. In addition 29 CFR 1910.1000 contains a computational approach to assess exposure to a mixture. This calculation should be used when components in the mixture pose a synergistic threat to worker health.

Whether using a single standard or the mixture calculation, the SAE of the individual constituents must be considered before arriving at a final compliance decision. These SAEs can be pooled and weighted to give a control limit for the synergistic mixture. To illustrate this control limit, the following example using the mixture calculation is shown. The mix ture calculation is expressed as:

... where:

•Em _ equivalent exposure for a mixture (Em should be _ 1 for compliance)

•C _ concentration of a particular substance

•L _ PEL

For example, to calculate exposure to three different, but synergistic substances:

Material 8-h exposure 8-h TWA PEL (ppm) SAE

Substance 1 500 1000 0.089

Substance 2 80 200 0.11

Substance 3 70 200 0.18

Using Equation I: Em _ 500/1000 _ 80/200 _ 70/200 _ 1.25

Since Em _ 1, an overexposure appears to have occurred; however, the SAE for each substance also needs to be considered:

• Exposure ratio (for each substance): Yn _ Cn/Ln

• Ratio to total exposure: R1 _ Y1/Em1 ...Rn _ Yn/Em

• The SAEs (95% confidence) of the substance comprising the mixture can be pooled by:

• The mixture control limit (CL) is equivalent to 1 _ RSt.

-If Em _ CL1, then an overexposure has not been established at the 95% confidence level; further sampling may be necessary.

-If Em _ 1 and Em _ CL1, then an overexposure has occurred (95% confidence).

• Using the mixture data above:

Therefore Em _ CL and an overexposure has occurred within 95% confidence limits.

This calculation is also used when considering a standard such as the one for total welding fumes.

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