I am new to workplace monitoring and am struggling a bit with the calculation. We recently had some monitoring done for expsoure to petroleum ether fumes. We used a diffusion badge and had the testing (for total hydrocarbons) done by an idependant laboratory the results were as follows. Measured quantity was 3210 ug Exposure was 309mg/m3 (based on open badge for 7hrs 40mins) Lab calculated to 8Hr TWA as 296mg /m3 I think I understand how these figures were arrived at but after some discussion there is some feeling that we have made a poor assumption. While the badge was open for 7:40 actual exposure can be confined to a period of 75 mins in 6 sessions of 10-20 minutes. When I try to use example 4 in EH40 I come up with a completely different average of 48.3mg/m3 (309 x 75/480. I ignored 'non exposure' time and converted to minutes for ease of calculation. It has now been suggested that short term exposure could in fact be significantly higher (approximately 2880mg/m3. However I have not seen any calculation to show how this figure was achieved and I can't understand how it could be so high if the total exposure was only 309mg/m3. I may be missing something obvious but any help would be appreciated. Thanks