TLVs for heat stress

This week we’re running the BOHS module M201 Thermal environment and non-ionising radiation (including lighting). This is one of the optional modules and most hygienists will only come across problems related to these topics on rare occasions. Consequently, after the course, it can be difficult to keep up to date with new research and developments.

One of the important aspects of heat stress covered on the course are the standards used when evaluating the risk. There are no legal limits in the UK relating to work in hot environments, so most hygienists will turn to the threshold limit values (TLVs) set by the American Conference of Governmental Industrial Hygienists (ACGIH) for guidance when faced with a potential heat stress problem. In the past the TLVS have often been used to establish work:rest regimes for work in hot conditions, as restricting working time is a practical measure that can allow work to take place while minimising the risk to the employees’ health. Unfortunately the emphasis placed on this organisational measure, meant that other, more effective, approaches such as looking for ways to avoid work in stressful conditions and engineering controls to reduce heat stress at source or along the transmission path, could be neglected

The TLVs were, however, updated a few years ago and now place less emphasis on work:rest regimes, adopting a more thorough, structured approach to reducing and managing the risks from work in hot environments. ACGIH have produced a flow chart that summarises the new procedure (a copy can be downloaded from Professor Thomas E. Bernard’s website  here or click on the diagram below), but it is fairly complex can be difficult to follow at first.


I’ve produced a summary presentation on the TLV, which includes some worked examples for use on our course which I’ve uploaded to Slideshare . I’ve embedded it below, but you can view it on the Slideshare site here,  from where it can also be downloaded, if you prefer.


Is it really necessary to memorise exposure limits?

Many years ago, on the day I started work, after my new boss had run through the company induction, shown me around the department and told me as much as he could think of about the mysterious world of occupational hygiene I’d just joined, he’d clearly had run out of ideas about what to do with me. So he gave me a copy of the current list of occupational exposure limits (in those days HSE used to reproduce the American TLVs®*) and told me to read through it. Looking at the long list of substances I remember asking him whether I was expected to learn them. “Of course not!” was his answer. “You can always look up a limit if you need it”. What he wanted me to do was to read through the accompanying explanatory material so that I would understand what the limits were, their legal status and how to apply them.

Given that there are several hundred limits it would be a nonsense trying to memorise them. Although as an A4 publication the HSE’s list of Workplace Exposure limits, EH40, is too big to fit in your pocket (the TLV list is much handier and pocket sized), it’s easy enough to keep in your briefcase or kit bag. Even if you leave it in the office, it generally wouldn’t take too long to find the limit you need in most cases.

These days it’s even easier to check the list as EH40 is now published online by the HSE  and can be downloaded as a pdf file here free of charge. I keep copies on my laptop and on my phone so I can consult it whenever I need to.

Of course, most hygienists over the course of time will tend to memorise some limits – for substances they come across regularly. Even then it can be dangerous to rely on memory. Limits can change and you can find yourself quoting an out-dated limit if you’re not careful. Which could lead to misinterpretation of sampling results and inappropriate advice on controls if you don’t double check the limits first.

*TLVs® – the Threshold Limit Values published by the American Conference of Governmental Industrial Hygienists