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UNIT 9
SUPPLEMENTARY HYGIENE SAMPLING
AND COMPLIANCE INFORMATION
BASIC DESCRIPTION OF VARIABLES USED IN HYGIENE
CALCULATIONS AND SAMPLING CONSIDERATIONS
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
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
FLOW RATE
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 diff erence to calculate duration
SAMPLE DURATION
The volume collected can be determined by using the sample flow rate and sample duration
NOTE: Volume will most likely need to be converted to m3, which
can be done either before entering into concentration equation or after
VOLUME COLLECTED
If we multiply the flow rate by duration we can see that we cancel out minutes and are left with liters
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 (µg) nanograms (ng)
MASS OF SUBSTANCE
Concentration of a substance is calculated using the volume collected (previously calculated) and the mass reported by the laboratory
Incorporating flow-rate formula we get an overall formula:
CONCENTRATION
Sample ID
Start Time
End Time
Sample Duration(min)
Pre flow rate (l/min)
Post flow rate (l/min)
Average flow rate(l/min)
2001 8:02 4:20 1.998 1.967
2051 8:05 4:01 1.965 1.725
2053 9:10 4:32 1.971 1.968
SAMPLE CALCULATION (STEP 1: CALCULATE SAMPLE DURATION/FLOW
RATE)
= (4:20 pm – 8:02 am) = (16:20 – 8:02) = 8 hours + 18 min = 480 min + 18 min = 498 minutes
Where,8 hours * (60 min/hour) = 480 min
SAMPLE CALCULATION (STEP 1: CALCULATE SAMPLE DURATION/FLOW
RATE)
Sample ID
Start Time
End Time
Sample Duration (min)
Pre flow rate (l/min)
Post flow rate (l/min)
Average flow rate(l/min)
2001 8:02 4:20 498 1.998 1.967
2051 8:05 4:01 476 1.965 1.725
2053 9:10 4:32 442 1.971 1.968
= (1.998 l/min + 1.967 l/min) 2= (3.965 l/min) / 2= 1.982 l/min
𝑓𝑙𝑜𝑤𝑟𝑎𝑡𝑒=(𝑝𝑟𝑒 𝑓𝑙𝑜𝑤𝑟𝑎𝑡𝑒+𝑝𝑜𝑠𝑡 𝑓𝑙𝑜𝑤𝑟𝑎𝑡𝑒)
2
Take smaller fl ow rate and multiply by 10%/5%: 1.967 l/min * 0.1 = 0.197 l/min
Check to ensure other fl ow rate is within 10% 1.967 l/min + 0.197 l/min = 2.164 l/min (OK)
Check fl ow rate within 5% 1.967 l/min * 0.05 = 0.098 l/min + 1.967 l/min = 2.065 l/min (OK)
Sample ID
Start Time
End Time
Sample Duration (min)
Pre flow rate (l/min)
Post flow rate (l/min)
Average flow rate(l/min)
2001 8:02 4:20 498 1.998 1.967 1.982
2051 8:05 4:01 476 1.965 1.725 1.845
2053 9:10 4:32 442 1.971 1.968 1.970
SAMPLE CALCULATION (STEP 2: CHECK FLOW RATES WITHIN 10 & 5
%)
Pre and post flow rates for samples 2001 and 2053 are within 5% of each other Valid Samples
Pre and post flow rates for sample 2051 are not within 10% of each other invalid sample (Throw out)
SAMPLE CALCULATION (STEP 2: CHECK FLOW RATES WITHIN 10 & 5
%)
Sample ID
Start Time
End Time
Sample Duration (min)
Pre flow rate (l/min)
Post flow rate (l/min)
Average flow rate (l/min)
2001 8:02 4:20 498 1.998 1.967 1.982
2051 8:05 4:01 476 1.965 1.725 1.845
2053 9:10 4:32 442 1.971 1.968 1.970
= (1.982 l/min * 498 min)
= (1.982 l/min * 498 min)
= 987 liters
Convert to m3 = 987 liters * (1 m3/1000 l)
= 0.987 m3
SAMPLE CALCULATION (STEP 3: CALCULATE VOLUME m3)
Sample ID
Sample Duration (min)
Average flow rate (l/min)
Volume (liters)
Volume (m3)
Mass (mg)
Concentration (mg/m3)
2001 498 1.982
2051 476 1.845
2053 442 1.970
= (2.54 mg)/(0.987 m3)= 2.57 mg/m3
SAMPLE CALCULATION (STEP 4: CALCULATE CONCENTRATION mg/m3)
Sample ID
Sample Duration (min)
Average flow rate (l/min)
Volume (liters)
Volume (m3)
Mass (mg)
Concentration (mg/m3)
2001 498 1.982 987 0.987 2.54
2051 476 1.845 Na Na Na
2053 442 1.970 871 0.871 1.89
*Na = Not applicable
SAMPLE CALCULATION (FINAL CONCENTRATIONS)
Sample ID
Sample Duration (min)
Average flow rate (l/min)
Volume (liters)
Volume (m3)
Mass (mg)
Concentration (mg/m3)
2001 498 1.982 987 0.987 2.54 2.57
2051* 476 1.845 Na Na Na Na
2053 442 1.970 871 0.871 1.89 2.17
FIELD BLANKS
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 fi eld blanks? To test for contamination of samples during transportation, handling,
and storage
How many fi eld 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
What do you do if mass is reported on field blanks? 1. 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
2. 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
3. Ignore contamination and include all samples It is recommended not to use this option bad practice
HOW TO TREAT FIELD BLANK RESULTS
COMMON REASONS PEOPLE DO NOT TAKE FIELD BLANKS
1. Don’t know they should Many people taking hygiene samples lack training on proper
sampling collection procedures and best practices
2. Don’t 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 reputation viewed as doing “bad job”/inadequate performance
Feel like all the work was done for nothing not completing tasks
3. Budget restraints Often budgets for hygiene sampling is very limited and people do
not want to allocate a significant proportion (~10%) to “blanks”
What does it mean if we fi nd 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 don’t have fi eld blanks) We can overestimate exposures May lead to:
1. Additional sampling (probably more costly than including 10% blanks)
2. Implementation of potentially unnecessary controls (very costly)
3. Workers’ compensation orders for non-compliance
In summary, fi eld blanks: Increases our confidence in our measurements Saves time and money
HOW TO ‘SELL’ FIELD BLANKS
LIMIT OF DETECTION
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 diffi cult to deal with and interpret
LOD DEFINITION
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/√2) 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
DETERMINING COMPLIANCE FROM
EXPOSURE DATA
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 confi dence limits
For this topic please recall readings from last week that covered confi dence limits and determination of compliance (pg. 510-512 of text) and also readings from this week (pg. 516-517)
DETERMINING COMPLIANCE
The fi rst step to determine compliance is to calculate the upper and lower confi dence 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 “true” 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
Confi dence limits are limits placed around the mean (i.e. average) that represents the amount of uncertainty in our samples
The confi dence 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 confi dence limit ↔ lower confi dence limit) specifi es the range of values in which the true exposure mean may lie at a specifi ed confi dence 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
The confi dence limit method used to determine compliance compares the mean, upper and lower confi dence limits to the exposure limit
If the upper confidence limit is below the exposure limit we can say that we are complaint “on average”
If the lower confidence limit is above the exposure limit we can say that we are not compliant “on average”
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
COMPLIANCE CHART
Exposure Limit
Conce
ntr
ati
on
Compliant Possibly non-compliant Non-Compliant
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