8. Processing measurement data
There are many strategies to evaluate occupational exposure to chemicals by means of a random sample of measurements; one requires some more measurements than the other,but the ultimate goal is to analyze the collected measurement data and to make a statement regarding the actual exposure of the workers’ population, and whether the pre-set limitvalue is respected.
- NBN EN 689:1995 Workplace atmospheres - Guidance for the assessment of exposure by inhalation to chemical agents for comparison with limit values and measurement strategy
- NVvA-BOHS Testing Compliance with Occupational Exposure Limits for Airborne Substances suggestion for an alternative assessment strategy for NBN EN 689:1995
BWStat is an excel tool designed to facilitate the implementation of the BOHS-NVvA Guidance. The tool allows the estimation of, from three or more measurements, group and individual compliance with the OEL, taking into account the intra- and inter-individual variation in repeated measurements from the same individuals, and measurements in different individuals from a similar exposure group. By following the BOHS-NVvA Guidance, BWStat is much more advanced than IHStat for example. Version 1 was launched during the NVvA conference 2013 and works using either Dutch, English, German or French. Version 2, additionally available in Spanish, was launched during the IOHA congress 2015 by means of a Personal Development Course at the start of the conference, and a poster during the conference. The tool was developed by Tom Geens (Provikmo & BSOH), Theo Scheffers (TSAC, NVvA & BSOH) and several other colleagues from Belgium and abroad. The presentation, the poster and the tool itself are free to download. A manual is under development.
Do you have (repeated) measurements and you want to verify if your measurements follow a (log)normal distribution, or do you want to calculate the exceedance probability regarding the limit value ? This can be done using the Excel sheet IHSTAT. Different versions of this tool are available on the internet. Depending on the version, this spreadsheet allows you to calculate the statistical key figures for 3 to 200 measurements of substances in the workplace. The use of this spreadsheet requires some knowledge of statistics. If you do not have this, you can gain some through Appendix 4 of the book "A strategy for assessing and managing occupational exposures", third edition, 2006. John Mulhausen built the original tool and together with the AIHA EASC (Exposure Assessment Strategies Committee), the tool was further improved. Since 2007, some other languages are supported, in addition to English (see presentation D. Drolet) and for most parameters also text balloons with explanations are provided. BSOH, together with the AIHA EASC, very recently implemented Dutch. This latest version of IHSTAT is available on the AIHA website since January 6, 2012. Open the file, enable macros, choose the language Dutch / Français / English / ... and click on the blue arrow. You can find a detailed example below ?? and boxes with explanations appear as one moves over the various calculated parameters.
Exposure Assessment Solutions Inc. is a U.S. company led by dr. Paul Hewett. Several freeware programs to estimate the exposure are available for download after registration.http://www.oesh.com/software.php. IHDataAnalyst is a kind of variant of the popular IHStat (also includes censored data analysis and Bayesian decision analysis). There is a free Lite version (up to 25 measurements) and an extensive paid version (up to 2000 measurements). These tools are available in English.
While the popular IHstat is used to calculate exceedance probabilities for homogeneous exposure groups (no intra-individual variation is taken into account), SPEED focuses on overexposure, which is not the same as exceedance. SPEED takes into account repeated measurements for the same person (intra-individual variation) as well as the differences within the sample group (inter-individual variation). SPEED can be obtained from the IRAS (English version).
HYGINIST is a specially developed statistical software tool for industrial hygiene purposes. It analyses the measurement outcomes obtained using the standard occupational hygiene strategy and compares them with the limit value. It plots on screen the distribution of 3 to 2000 measurable outcomes, as if on logarithmic probability paper. Also the match with the log-normal distribution is being tested. It helps in planning a cost-effective measurement program using unbiased estimators for the MAC exceedance probability and extracts maximum information from results below the detection limit. The program is available in Dutch and English.
On the homepage of INRS (French), "Altrex" can be typed into the search bar, and this will get you to the download page (French). In Altrex, data can be entered in any order (e.g. where the samples were taken and on which days). Altrex also deals with samples below the limit of quantification (LOQ). The output are graphs that can be easily pasted into a report. Currently the program is only available in French.