Short-Period Air Sampling by Direct-reading Instruments

Transcription

1 Short-Period Air Sampling by Direct-reading Instruments OHAO Meeting, Toronto, October 13, 2011 Ian Drummond and Paul Bozek, Dalla Lana School of Public Health, University of Toronto

2 Program for today's talk An Overview of the concept The results of this particular project A standardized method for using a DRI Accuracy and precision of the method A standardized evaluation of DRI results Further developments

3 The Overall Objective of the Project To simplify measurement of short-period exposure to chemicals so that it may be routinely and cheaply measured at the same time as a full-shift measurement is made. How cheaply? Why make measurements routinely? How?

4 How Cheaply? When compared to a TWA survey No extra field visits or field time needed. No extra disruption of the worker No extra laboratory charges If a $5,000 instrument is used for 3 years, measures 50 shifts per year $33 per shift $1 per 15 minute period

5 Why measure routinely? Most chemicals have a short-period OEL Ceiling STEL in conjunction with TWA Excursion in conjunction with TWA Complaints about chemical exposures are often associated with short, intense episodes Capturing the whole shift gives representative, not "worst-case", short-period data

6 Why measure routinely (2)? Better understanding of workplace conditions Evaluations of controls Short-period data will identify time and location Evaluation of variability of exposure Between people, tasks, production volume, etc. Anticipation of changes to exposure standards Beryllium and the US Dept of Energy

7 Beryllium 1996 the Beryllium TWA TLV was 2 µg/m a STEL of 10 µg/m 3 was added The US DOE had no idea if it met the new standard despite several thousand personal TWA samples R.M.Tuggle, Appl.Occup.Environ.Hyg. 15(4): , 2000 The STEL standard was unexpectedly stringent

8 Why measure routinely (3)? Research technology Solvents can reach the brain in as little as 2 minutes after exposure starts. This is possibly associated with headache. To address such issues, standardized, accurate methods of short-period measurement are needed, along with a lot of data (Think, "Cheap"!)

9 How to make Short-period measurements? What is the current method for organic vapours? Alternative methods using directreading instruments (DRIs) Photoionization detectors Barriers to use of alternative methods

10 Current method to measure Short-periods for organic vapours Pump and absorbent tube 15 minute samples for STELs Up to 30 minutes for excursions As short as possible for Ceilings Expensive (many samples) Intrusive (stop work to change tubes) Not representative (worst case scenarios) Specified in Govt. regulation

11 Alternative Method Direct-reading instruments Photoionization Detectors Stable Sensitive Respond to a wide range of materials Record measurements all shift Not selective Need specific calibration for specific materials Interferences are common Positive, other organic materials Negative, humidity, methane

12 Barriers to Adoption of DRIs The existence of a method (pump & tube) specified by Govt. regulation Positive and negative interferences Lack of standardized methods of use Measurement frequency and averaging time, in particular Lack of accessible statistical tools to evaluate exposure DRIs can produce thousands of data-points

13 Overcoming the Barriers Develop a standardized method (The C-tube correction method) to improve selectivity and accuracy of PIDs On-the-fly calibration of PIDs, I.Drummond Am.Ind.Hyg.Assoc.J. 58(11), , 1997 Compare the accuracy and precision of the PID method to the pump & tube method. Provide a method to evaluate results

14 The Basic Procedure 1. Make a TWA measurement using the standard pump-and-tube method 2. For exactly the same period, use a PID to record measurements 3. Calculate C-tube correction factor CF 1. CF = TWA (Pump/tube) / TWA PID 4. Correct individual PID measurements 5. Calculate 15 min average values for the workshift

15 Project procedures Styrene test material TWA 20 ppm; STEL 40 ppm Use a sequential sampler to collect eight 15 min. pump-and-tube samples for two-hour periods Use four PID-and-TWA samplers to measure the same 8 x 15 minute periods Two MiniRae 2000 and Two ToxiRae units Five runs in the laboratory and 15 runs in a workplace making plastics Acetone present as a clean-up solvent

16 The Laboratory

17 Results in the Laboratory Run 5, Time adjusted :19:12 16:48:00 17:16:48 17:45:36 18:14:24 18:43:12 19:12:00 19:40:48

18 Laboratory Results When the PID is calibrated using isobutylene as the standard, to convert to Styrene values use either: The manufacturer's documented value of 0.40 The C-tube correction factor derived from the simultaneous TWA sample For each 15 minute period the PID value was divided by the corresponding pump-and-tube value, giving an expected value of 1.00

19 Comparison of PID with Pump-and-tube in Laboratory in constant levels of styrene Manufacturer response 0.40 C-tube correction Mean Value Std.Dev No. samples

20 Comparison of PID with Pump-and-tube in Workplace with acetone present Manufacturer response 0.40 C-tube correction Mean Value Std.Dev No. samples

21 Conclusion on short-period measurement by PID By its self, a PID does not have adequate accuracy and precision. By use of the C-tube correction, accuracy is equivalent to pump-and-tube methods. Precision is equivalent in the laboratory. In the field, precision is lower, but the much larger quantity of data will compensate

22 Standardized procedure for short-period measurements Important timing parameters The instrument response time Characteristic of the PID The frequency of measurement Under control of the experimenter The desired sample averaging time Determined by the exposure standard

23 Instrument response time Often expressed by manufacturers as "time to 90% response" Implication is, "faster is better" Can be modeled as exponential rise and fall to a step change in air concentration. Example Miran 1A IR analyzer

24 Miran 1A flow characteristics Cell volume = 5.4 litres Sample flow = 2 l/min Using ventilation terminology Air-changes per sec = sec -1

25 Miran 1A response time The average residence time for air in the cell is 1 / (air-changes per sec) = 1 / = 162 seconds The air entering the cell is physically averaged by mixing inside the cell A reading from a Miran 1A looks back in time for 162 seconds The instrument response time of the Miran 1A is 162 seconds.

26 Evaluation of other instruments The technique of challenging an instrument with a step-change in air concentration is widely applicable All direct-reading instruments have a characteristic response time, and hence look back in time when making a measurement

27 Evaluation of PID response time ToxiRae 4273, 6th challenge Calculated Actual

28 Measured Response Times The equation of the rising trace is: C = C o ( 1 e -kt ) The instrument response time is 1/k MiniRae 2000 ToxiRae 0.5 seconds 2.5 seconds Electrochemical approx. 20 seconds Miran IR 162 seconds Colorimetric tubes have a sample averaging time of approx. 60 seconds per stroke

29 The measurement frequency DRIs might make measurements at time increments that are: Less than the instrument response time Over-sampling the signal Greater than the response time Under-sampling the signal The Nyquist rate is best Sample twice during every instrument response time (just like digitizing an audio signal)

30 Sample averaging time The desired sample averaging time is determined by the exposure standard to be used for comparison Multiple readings from a DRI, covering the desired time, must be averaged Some instruments have a setting to do such averaging internally, saving only the averaged value Under-sampled data will be more variable than over-sampled data

31 The effect of different averaging times The effect of averaging blocks of data is illustrated in the following lognormal probability plot The Geometric mean increases The Geo.std.dev. decreases The mean necessarily stays the same The starting data-set is lognormally distributed with GM = 1 and GSD = 2.713

32 The effect of averaging blocks of data, N = 1, 2, 8 and

33 How to compare two different instruments If a Minirae and ToxiRae made a series of simultaneous measurements Because the instrument response times differ by a factor of 5 The GMs would differ by a factor of 1.5 The GSDs would differ by a factor of 0.6 The mean values would be the same They would predict a different probability of exceeding a given limit (95% percentile)

34 How to compare two different instruments (2) To get comparable results, the sample averaging times MUST be the same 1. Set the instrument to record data at intervals equal to half the instrument response time (or faster). 2. Calculate the average value corresponding to the exposure standard

35 Sound-level meters An example of an instrument with a well-defined response time is a soundlevel meter (slow and fast settings). It would not be possible to get comparable readings from meters with different response times We are re-discovering this effect for chemical sensors.

36 How to analyze field data? Run 6, Time adjusted :43:12 13:12:00 13:40:48 14:09:36 14:38:24 15:07:12 15:36:00

37 Convert it to this! Instrument response factor 3.00E-01 calculated on "raw data" sheet Enter 8-h TWA for substance ppm Enter 15 min STEL ppm No. logging periods in 15 mins 180 Note: Check this calculated value against the actual sample period % STEL ToxiRae 4273 Run 6 and Time :15 13:30 13:45 14:00 14:15 14:30 14:45 15:00 15:15 15:30 15:45 16:00 16:15 16:30 16:45 17:00 17:15 17:30 17:45 18:00 18:15 18:30 18:45 19:00 19:15 19:30 19:45 20:00 20:15 20:30 20:45 21:00 Start of graph is time shifted forward by 0 minutes "ctrl a" to shift phase of graph by +120 o ppm is the 8-hour TWA (assuming no exposure in unmonitored periods) 0 15 Minute period exceeds the STEL 1 15 Minute periods exceeds the 8-h TWA OEL value 0 Periods less than 1 h between periods exceeding the 8-h TWA value

38 One Page with all the information you need! The C-tube TWA value Measured by the NIOSH method The calculated 8-hour TWA value The 4 criteria STEL evaluation Failed criteria are highlighted in red The ability to shift the 15 minute periods by 5 or 10 minutes

39 Excursions and Ceilings Similar one-page analysis exists for analyzing data against excursion limits and ceiling limits However. The ACGIH TLVs do not specify the sample averaging time for excursions and ceilings

40 Excursions Based on a careful reading of the scientific literature, and discussion, we are using the following definition Excursions in worker exposure levels may exceed 3 times the TLV-TWA for no more than two 15-minute periods during a workday, and under no circumstances should any 15-minute period exceed 5 times the TLV- TWA, provided the TLV-TWA is not exceeded.

41 Ceiling Limits Historically grab samples were used Pump and tube 10 min Colorimetric detector tubes 1 min Electrochemical cells 30 seconds Physiologically One breath 10 seconds Brain clearance half-life 1 minute

42 Ceiling Limit averaging time The criterion for evaluation is that no one minute period during the work-day should exceed the Ceiling OEL. PLEASE, do not use an instrument to evaluate ceiling exposures without knowing the instrument response time

43 Documentation Report on WSIB Project #09025 Includes step-by-step Guidelines for use of DRIs and charcoal tubes to simultaneously measure TWA and shortperiod exposures Excel spreadsheet for analysis of data References Details of the sequential sampler

44 Further work We don't know how to predict the probability of exceeding a STEL We don't know under what conditions the 8-hr TWA or the short-period limit is more controlling Why two standards for different times? We can't predict exposures at one time from data taken at a different averaging time Hence the Beryllium example where adding a STEL to a TWA had an unknown effect

45 How to address these questions? A robust statistical model of short-period data is perhaps now possible Autocorrelated data requires time-series analysis. This technique is now being extended to series with non-gaussian distributions DRIs can provide the large data-sets that are required for time series analysis, hundreds or thousands of values (8 hr at once per second is 4,800 values)

46 Summary of Talk The barriers to use of DRIs were reduced by: 1. Showing that use of the C-tube correction makes DRIs comparable in accuracy and precision to pump-and-tube methods, despite the presence of interference 2. Documented a standard procedure for use of the C-tube correction method, taking instrument response time into account 3. Provided a spreadsheet to use the C-tube correction method and to automate analysis of the data sets for STEL, excursion or ceiling limits