EH40 used to set out the method (page 50/51 or thereabouts as I recall -but that was the 2002 edition). You haven't mentioned the concentration of the dichloromethane in that product. It's nasty stuff.

Dichloromethane - yikes, I would really consider another material for your process, particularly if spraying the product But if you can't change material at present there is some detailed guidance for paint-stripping processes at the HSE, download this: http://www.hse.gov.uk/pubns/wis19.pdf Also known a Methylene Chloride, or DCM, it is a Category 3 carcinogen, which means 'possible' carcinogen, in that it has some effects that are of concern, so you really need a local ventilation system and I would NOT rely on 'caculating' ppm exposure; you need an effective meter/detector method of monitoring ppm in the workplace atmosphere to ensure the local ventilation is always effective. JohnW

Before we get too far, please check what substance you actually have. Four separate names have been used so far on this topic, and they relate to two completely different chemicals. Dichloromethane Cas no. 75-09-02 Methylene chloride, also Cas no 75-09-02 empirical formula CH2Cl2 1,1- Dichloromethane - does not exist. The reason is that this is also dichloromethane and it does not require the 1,1- designation as the chlorine can only be in one place in this molecule. however, 1,1- Dichloroethane does exist and is Cas no. 75-34-3, empirical formula C2H4Cl2 Which one do you have?

So what is the balance? This could be important, as pointed out by Leadbelly.

Another item of essential information is how you are spraying this. Is it using an aerosol can? If so, what is the propellant? This could have a bearing on both the airborne exposure and the skin exposure. Also please note that the information on gloves that I posted relates only to the single substance. Once you start to deal with mixtures it gets very much more complicated. For example I can show you a glove that with each of toluene and methanol has a permeation breakthrough time (according to the EN374-3 test) of >240 minutes but when used against a 1:1 mixture of the two the permeation breakthrough time drops to 9 minutes! You will also need to train the glove users so that they do not contaminate their hands when removing the gloves. This is a common problem. It is not uncommon when training to find that over 50% of glove wearers contaminate their hands when removing their gloves, even though they have been shown how. Chris

Purely from a theoretical perspective, and to try and answer the original question, and assuming this is a classroom based theory question rather than getting involved too deeply in assumptions about worker health, morals and the carinogen directive etc etc....: The TWA is an 8 hour average Actual usage is 2hours per day Assumption is that remaining 6 hours exposure is 0ppm So based on the 8hour TWA the maximum would be: 400ppm x 2hours / 8hours = 100ppm 8hr TWA So theoretically you could be exposed up to 400ppm for 2hours in an 8hour period which gives the TWA of 100ppm but of course this exceeds the 15minute STEL, dosn't it??? But wait.... You are only using the material for 10 out of that 15 minutes, averaging this out over 15minutes means that your 10minute exposure can be up to 450ppm to still not exceed the 15minute STEL assuming 0ppm for the other 5 minutes. Because... 450 x 10mins / 15mins = 300 Sooooo theoretically, im speaking theoreticaly here.... Yes you can be exposed to 400ppm for 2 hours a day in 10 minute intervals. Des

Just a further note of caution and clarification to Descarte's theoretical response at #12 above. In practice, this would be contrary to Regulation 7 of the COSHH Regulations.