Unit 9 hygiene calculations sampling issues compliancePresentation Transcript
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Unit 9Supplementary hygiene Sampling and compliance information
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Basic description of variables used in hygiene calculations and sampling considerations
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Flow rate is the rate of which air is being pulled through the sampling device Typically reported as liters/min (l/min) Calculate average between pre and post calibration measures ππππ€πππ‘π=(πππΒ ππππ€πππ‘π+πππ π‘Β ππππ€πππ‘π)2 NOTE on calibration: Pre and post measurements must be within 10% or sample is invalid and should be thrown out If >5% but <10%, sample may be considered with caution Β Flow Rate
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Sample duration is the total length of time the sample was collected Typically this is reported in minutes (min) but can also be reported in seconds, hours, days, or weeks During measurement record the (1) start time and date when sampling begun, (2) the end time and date when sampling ceased Take the difference to calculate duration ππ’πππ‘πππ=Β πππΒ π‘πππΒ βπ π‘πππ‘Β π‘πππ Β Sample duration
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The volume collected can be determined by using the sample flow rate and sample duration π£πππ’ππ=ππππ€Β πππ‘πΒ βππ’πππ‘πππ π£πππ’ππΒ πππ‘πππ =πππ‘πππ ππππ’π‘πβππππ’π‘ππ π£πππ’ππΒ πππ‘πππ =πππ‘πππ ππππ’π‘πβππππ’π‘ππ NOTE: Volume will most likely need to be converted to m3, which can be done either before entering into concentration equation or after Β Volume Collected If we multiply the flow rate by duration we can see that we cancel out minutes and are left with liters
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For most analytical methods we will be provided with a mass value from the analytical laboratory that conducted the analysis of the samples The units will depend on the measurement method Common unit values would include: grams (g) milligrams (mg) micrograms (Β΅g) nanograms (ng) Mass of substance
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Concentration of a substance is calculated using the volume collected (previously calculated) and the mass reported by the laboratory πΆππππππ‘πππ‘πππ=πππ π π£πππ’ππ=πππππ‘ππ Incorporating flow-rate formula we get an overall formula: πΆππππππ‘πππ‘πππ=πππ π ππππ€πππ‘πβππ’πππ‘πππ=πππππ‘πππ ππππ’π‘πβππππ’π‘ππ Β Concentration
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π¬ππππππΒ πΊπππππΒ π°π Β ππππ Take smaller flow rate and multiply by 10%/5%: 1.967 l/min * 0.1 = 0.197 l/min Check to ensure other flow rate is within 10% 1.967 l/min + 0.197 l/min = 2.164 l/min (OK) Check flow rate within 5% 1.967 l/min * 0.05 = 0.098 l/min + 1.967 l/min = 2.065 l/min (OK) Β Sample calculation (step 2: Check flow rates within 10 & 5 %)
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Pre and post flow rates for samples 2001 and 2053 are within 5% of each other ο Valid Samples Pre and post flow rates for sample 2051 are not within 10% of each other ο invalid sample (Throw out) Sample calculation (step 2: Check flow rates within 10 & 5 %)
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*Na = Not applicable Sample calculation (Final concentrations)
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Field blanks
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Field blanks are samples that are sent out during sampling that are opened and closed without pulling air through them What is the purpose of field blanks? To test for contamination of samples during transportation, handling, and storage How many field blanks should you use? It depends but recommended practice is 10% of your number of samples Do we have to analyze the samples? YES you must! Best practice Field blanks
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What do you do if mass is reported on field blanks? Throw the samples out for that sampling period Good option if contamination is limited to small number of samples or if contamination levels were high Adjust for the contamination Acceptable if contamination levels are not too high If small batch is contaminated we can adjust only those samples from the contaminated batch by the field blank value If contamination is on multiple blanks during a sampling project we can adjust for each batch or we can apply an adjustment to all samples using average field blank value Ignore contamination and include all samples It is recommended not to use this option ο bad practice How to treat Field blank results
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Common Reasons people do not take Field blanks Donβt know they should Many people taking hygiene samples lack training on proper sampling collection procedures and best practices Donβt want to risk having to throw out samples Perceived risk of job Can be regarded as throwing money away in eyes of management Risk of reputationο viewed as doing βbad jobβ/inadequate performance Feel like all the work was done for nothing ο not completing tasks Budget restraints Often budgets for hygiene sampling is very limited and people do not want to allocate a significant proportion (~10%) to βblanksβ
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What does it mean if we find contamination in our blanks? We may potentially have contamination in our samples Our reported results may be higher than the actual exposure levels By having blanks we are aware of contamination and can adjust accordingly What does it mean if we had contamination and do not know (i.e. we donβt have field blanks) We can overestimate exposures May lead to: Additional sampling (probably more costly than including 10% blanks) Implementation of potentially unnecessary controls (very costly) Workersβ compensation orders for non-compliance In summary, field blanks: Increases our confidence in our measurements Saves time and money How to βsellβ field blanks
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Limit of detection
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What is LOD? LOD stands for the Limit Of Detection This is the lowest level (e.g. concentration) measureable by an analytical method or sampling device Why is this important Measurements under the LOD do not give us much information on the hazard but they cannot be ignored/omitted from analysis or the discussion of results Having multiple LOD measurements often results in skewed or lognormal data distributions They can be difficult to deal with and interpret LOD Definition
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Several methods have been proposed, most important thing to remember is you cannot omit them from determining the average concentrations. Two most commonly used: Method 1 Multiply the LOD by 0.5 (i.e. LOD/2) for each data point that was <LOD For example if the LOD reported is 2 ppm then you would input (2ppm*0.5 = 1ppm) Only use when the data are highly skewed (GSD approximately 3.0 or greater) Method 2 Multiply the LOD by 0.707 (i.e. LOD/β2) for each data point that was <LOD For example if the LOD reported is 2 ppm then you would input (2ppm*0.707 = 1.4 ppm) Use when data not highly skewed Methods to deal with <lod measurements
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Determining compliance from exposure data
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Now that we have conducted sampling how do we determine if we are compliant with the regulations? Do we compare each reading/sample with limits? Do we calculate the % of samples over the limits? Do we compare the average of the readings/samples with the limits? Although these methods are commonly used compliance is a bit more complex and methods for determining compliance are under debate For this class we are going to review a method frequently used and accepted in North America using confidence limits For this topic please recall readings from last week that covered confidence limits and determination of compliance (pg. 510-512 of text) and also readings from this week (pg. 516-517) Determining compliance
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The first step to determine compliance is to calculate the upper and lower confidence limits of the mean Why do we do this? When we take samples we introduce uncertainty/error into our measurement This comes from error in our measurement, instruments, and analysis This means the measurement we take is not the βtrueβ value of the exposure The true value is the measured exposure +/- error Calculating confidence limits (or the confidence interval) allows us to account for some of the error/uncertainty in our measurements Determining compliance using confidence limits
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Confidence limits are limits placed around the mean (i.e. average) that represents the amount of uncertainty in our samples The confidence limits include an upper and a lower bound estimate: LCL = lower confidence limit, the lower bound limit UCL = upper confidence limit, the upper bound limit This interval (upper confidence limit β lower confidence limit) specifies the range of values in which the true exposure mean may lie at a specified confidence level (95% most common) More narrow the interval, the more precise our measurements are More wide the interval, the less precise our measurements are Confidence limits
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The confidence limit method used to determine compliance compares the mean, upper and lower confidence limits to the exposure limit If the upper confidence limit is below the exposure limit we can say that we are complaint βon averageβ If the lower confidence limit is above the exposure limit we can say that we are not compliant βon averageβ If the lower and upper confidence limit crosses the exposure limit it is unclear if we are compliant or not and require further testing Using confidence limits to determine compliance The next slide graphically displays the concept where: Upper Confidence Limit Mean Lower Confidence Limit