Evaluating cold environments

snowy wrightington

Just a couple of days ago it was the first day of Spring. But it’s more like the middle of winter today in the north of England with heavy snow falling outside which has been blown around by a fairly strong wind. Perhaps the weather is appropriate as this week we’ve been running the BOHS/OHTA Module W502 on the Thermal Environment. And although much of the course is concerned with hot environments, we also cover work in cold conditions.

In general, it’s usually easier to evaluate cold conditions as there are only two environmental parameters that need to be measured – the air temperature and air velocity. Humidity and radiant heat, which are important in hot environments, are less important in the cold. But evaluating the risk is more problematic. As with the heat there are indices we can use and standards we can refer to. The main ones being published by the American Conference of Governmental Hygienists (ACGIH) in their Threshold Limit Values (TLVs) for physical agents. These give work/warm up schedules for a variety of conditions. However as can be seen for the following table the TLVs are really only applicable to very severe conditions that can be encountered working outdoors in places such as Alaska and Canada in the Winter. In the UK outdoor temperatures never get down as low as those in the TLV table. And the coldest workplaces are probably frozen food stores where the temperature will not be less than –30 C with minimal air movement (although this can be, in effect, increased for personnel operating vehicles).

Threshold Limit Values Work/Warm-Up Schedule for Four Hour Shift

Source: CCOHS Canada. Adapted from Threshold Limit Values (TLV) and Biological Exposure Indices (BEI) booklet: published by ACGIH, Cincinnati, Ohio, 2008

So there really isn’t any guidance or standards that are applicable to more temperate climates like in the UK and most of Europe. So how can we evaluate the risk? I think that the answer is that it is mainly a case of using experience, professional judgement and common sense.

Some guidance on assessing cold environments, and an observation checklist, are provided in International Standards ISO 15265 (2000) Ergonomics of the thermal environment: risk assessment strategy for the prevention of stress or discomfort in thermal working conditions, and  ISO 15743 (2008) Ergonomics of the thermal environment – Working practices for cold environments. Unfortunately these, like most standards, are extremely expensive to purchase. There is quite a good paper in the Ingvar Holmér of Lund University in Sweden, Evaluation of cold workplaces: an overview of standards for assessment of cold stress, published in the Journal Industrial Health in 2008 and is available online here

Guidance from the UK Health and Safety Executive (HSE) on cold stress is very limited, referring the reader to various expensive International Standards.  There is a useful chapter in the ILO Encyclopaedia of Occupational Safety and Health chapter on Cold environment and cold work which can be accessed online.

In practice, options for controlling exposure to cold environments, whether outdoors or in workplaces such as food storage and preparation areas, are fairly limited. Appropriate clothing is inevitably going to be one of the main solutions.

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.

Thermal Comfort – A different sort of problem

Yesterday I travelled up to Glasgow where I’d been invited to give presentation on “Thermal Comfort” to a meeting of the Scottish Region of the British Occupational Hygiene Society (BOHS). Despite a few technical problems which delayed us starting (computers are great when they work!) the meeting went well and it was good to meet up with an enthusiastic group of people which included some new members of the Society.

Most occupational hygienists at some time or other will be faced with a situation where workers are complaining that the environment they’re working in is uncomfortable. Thermal conditions often contribute to this.

Thermal comfort is usually defined as “that condition of the mind that expresses satisfaction with the thermal environment” As this is about individual perception and preference, these problems are amongst the most difficult we have to address.


It’s most likely to be an issue in workplaces such as offices, but complaints or concerns can sometimes occur in manufacturing environments and other types of workplace. During the discussion, one of the audience told us about problems experienced by bus drivers – not surprising really when you consider that buses are effectively mobile workplaces with their own internal environment. Their large windows potentially presenting problems from radiant heat from the sun (or radiant heat loss on cold, sunless days) and there’s plenty of potential for draughts.

The key thing with “thermal comfort” is that it’s subjective – one person may complain about feeling warm while another may consider the same environment to be too cool – you can’t keep everyone happy all of the time. Studies have suggested that the very best you can do is achieve 95% satisfaction . So there will always be 1 person in 20 who is dissatisfied. Change things for him/or her, someone else will start to feel unhappy! In practice you’re doing well if more than 80% are happy!


There are two basic approaches we can take to evaluating thermal comfort

  • a Qualitative approach which involves talking to and interviewing people in the workplace and looking around for obvious clues, using experience to make a subjective judgement
  • a Quantitative approach where we carry out measurements. (However, interpreting the results from the measurements presents some difficulties)

I think that the best way to find out what someone thinks is to ask them! There are a number of questionnaires and checklists available from various sources, including the Health and Safety Executive and BOHS that can help with this qualitative approach. However, measurements can also be useful. They can help you to determine what factors are likely to be causing the problem. So, in practice, a combined approach is normally best.

Even if problems can be identified, it’s not always easy to find solutions. Particularly as improving things for one person is likely to upset someone else! Ideally, individuals should be given control over their own personal environment. But it’s not easy to achieve this in practice as changes that one person makes is likely to affect at least some of their colleagues.

The slides I used during the talk are available on Slideshare, including a version with my speaker’s notes.

Heat stress indices


Assessing the risk from the thermal environment is not easy. One of the main reasons for this is that there are a large number of factors which affect our physiological response to heat. The main ones are

Four environmental parameters

  • Air temperature
  • Mean radiant temperature
  • Relative humidity
  • Air velocity

And three “human” factors

  • Work rate (which affects metabolic rate) and
  • Clothing level
  • Whether the person is acclimatised to hot conditions

thermal env

All of these parameters* can be quantified, either by measurement or estimation, but it is very difficult to make sense of the results. We need to find a way of pulling them together so we can decide whether we have a problem and, if so, how big the problem is.

Over thirty different indices have been developed over the years, taking into account some or all of these parameters, which aim to integrate the relevant variables into a single value which relates to the stress placed upon workers by the thermal environment they encounter. These indices have differing degrees of success in meeting the objective of a single number descriptor for any situation. Each has a range of environmental parameters over which it is applicable, but outside of this range accuracy is questionable.

A number of these indices are listed in the syllabus for the BOHS Occupational Hygiene Module M201 “Thermal environment and non-ionising radiation (including lighting)”:

  • Effective Temperature (ET)
  • Corrected Effective Temperature (CET),
  • Wet Bulb Globe Temperature (WBGT),
  • Heat Stress Index (HSI) and
  • Predicted 4- Hour Sweat Rate  (P4SR)

Although they have to be covered during the course, these indices, except for WBGT, are out of date and, in practice, are no longer used (although ET is still applied in the mining industry in the UK and some other countries). In the main, they are of historical interest only.

The relevant indices for modern practice are WBGT and the more complex “Predicted Heat Stress” (PHS) index described in International Standard ISO 7933:2004 “Ergonomics of the thermal environment – Analytical determination and interpretation of heat stress using calculation of the predicted heat strain”. The latter is not mentioned in the current BOHS syllabus.

WBGT is easy to use and apply. The threshold limit values (TLVs) established by the American Conference of Governmental Industrial Hygienists (ACGIH) use the WBGT index. The TLVs also incorporate guidance on how to assess the risks from heat stress and strain, which include screening criteria based on the WBGT index and basic advice on  controls including work:rest regimen for different situations

PHS is extremely complex and intended to be used by experienced ergonomists or occupational hygienists.  The index determines a duration limited exposure time, which is the time taken for either the rectal temperature or the cumulative water loss reaches specified maximum values. The lower of the two values is used to determine the allowable exposure time. Solving the PHS equations requires the use of a computer program and should only be attempted by someone with appropriate experience.

In practice, for situations where there is a potential risk from heat stress and strain, it is best to adopt a structured approach.

  1. Undertake screening using a simple method that can be used by management and workers to identify potential risks from heat stress and strain and determine whether further work is needed. The HSE have a useful checklist that can be used during a screening survey that can be downloaded from their website.  Following the survey if it is clear that there is no risk no further action will be required. Also, if there are obvious problems it may be possible to specify appropriate controls at this stage. The HSE checklist can help with this.
  2. If the risk cannot be adequately assessed during stage 1, carry out a basic analysis taking appropriate measurements and interpreting them using WBGT. In many cases it will be possible to use the guidance provided in the ACGIH TLV for heat stress to determine whether the risk is significant and what controls and other measures could be implemented.
  3. For very complex cases, where it is not possible to adequately evaluate the risk during the previous two stages, commission an expert analysis which is likely to involve measurements/estimates of physical and/or physiological parameters and use of more complex indices.


* Other factors can also influence the risk too, such as posture, body weight, medication. A fully comprehensive analysis would consider them too, although it is not so easy to quantify them and including them introduces complications.

Heat Stress

The health risks associated with heat stress at work is a subject traditionally studied by occupational hygienists. In the past, in the UK, it was a compulsory component in the modular route to the Certificate examination. Currently it has been “relegated” to an optional module (although that may change in the future). However, questions about are still likely to be asked on heat stress on the Core exam and also in the Oral exam. Candidates for the latter may still be asked about the thermal environment even if they haven’t taken the optional module (M201), so its important that everyone taking the exam knows at least the basics.

Most hygienists don’t come across heat stress problems on a regular basis – it is more likely that they’ll have to deal with a thermal comfort issue. Yet there are still many industries where thermal stress can be a concern. These include traditional industries like steel making, glass manufacture, foundries, mining etc., but other, less obvious types of work may lead to workers being exposed to hot environments – e.g.catering,  bakeries, insulation contractors, firefighters, boiler room workers and anyone working outdoors in hot conditions.  Exposure is often transient and employers may not always identify and consider the risk adequately in their risk assessments. In many cases workers will not be acclimatised to hot conditions making them less tolerant to the heat.

Assessing the risks isn’t easy. There are seven main factors that influence human response to the thermal environment and in an ideal world all of them need to be considered.  Making sense of the data obtained from any measurements can be difficult.

As with most risk assessments it’s sensible to adopt a phased approach. If there is an obvious problem it’s better to look at the control options rather than worrying about what measurements to take. In cases of uncertainty, it may be possible to use the Wet bulb globe temperature (WBGT) index to make a basic evaluation of the risk as there are standards available against which the results can be compared (the ACGIH Threshold Limit Values for heat stress). In extreme cases extensive measurements followed by analysis using a more sophisticated index or even physiological measurements may be needed.

I’ve recently pulled together an introductory talk on heat stress, covering the hazards, measurement, evaluation and control, which I’ve uploaded to Slideshare. You can view it there, or in the embedded version below

Some useful material on heat stress is available on the HSE NIOSH and CCOHS websites

Photo credit: nicootje Stock.XCHNG

BOHS Meeting on the Thermal Environment

Working in extreme heat

We had a good turnout at the meeting today in Ellesmere Port  28 people  turned up to listen to four speakers who covered a number of aspects of managing the thermal environment.

Doug Hiebert talked about how BNFL dealt with a problem at their Selafield site.  Maintenance work has sometimes to be carried out in relatively high temperatures and high relative humidity and the risk is increased by the need to wear very comprehensive protective clothing to protect workers from the radiological hazards.  One problem they faced when assessing the risk is that it wasn’t possible to use the usual equipment to carry out environmental measurements due to the potential for it to become contaminated.

After assessing the problem a number of measures were implemented including

  • classifying the area as a “confined space” and introducing restrictions on acess based on factors that could affect an individual’s susceptibility to heat strain
  • trying to schedule maintenance work during plant shutdowns
  • introducing quite stringent and conservative time restrictions for work in the area
  • providing cooling vests

Andrew Moore from the HSE was the second speaker. He provided a regulatory perspective but also gave some good advice on how to manage the health risks associated with work in hot environments. He stressed the importance of obtaining competent advice and effective management, ensuring that all the relevant stakeholders are involved.  He illustrated his presentation with a case study where workers at a leisure pool were working in relatively high temperatures and high humidities. Although treated as a thermal comfort problem, the temperatures involved meant that the workers could be considered to be experiencing thermal stress.

I was the third speaker. Copies of my slides with notes can be viewed on Slideshare and a previous post.

The final speaker was Len Morris of the HSE who announced the formation of a BOHS Topic group on the thermal environment. Anyone interested in getting involved should contact BOHS HQ.

There are a number of good on-line information sources for anyone who’d like to find out more about managing the risks to health presented by the thermal environment, including the following

Thermal Comfort

The thermal environment is one of the traditional topic areas studied by occupational hygienists.  There are two issues:

  • thermal stress and strain
  • thermal comfort

Excessive thermal stress means that the body has to work hard to avoid he core temperature moving outside narrow limits (37 + 2 C).  If that happens we’re in a serious situation that  leads to serious health effects and may be fatal. Our efforts to prevent this happening can also lead to adverse effects.  This is most likely to occur in extreme environments or, sometimes, in more moderate environments where particularly heavy work is being performed or clothing is worn which prevent metabolic heat escaping.

Thermal comfort is most likely to be an issue in workplaces such as offices, but complaints or concerns can sometimes occur in manufacturing environments and other types of workplace. In this case the body is not experiencing a level of stress that it can’t cope with. There isn’t a physiological problem and ill health will not occur due to excessive thermal strain. But that encompasses a wide range of conditions. Will all of them be “comfortable”? Experience clearly shows that the answer to that is “no” !

Most occupational hygienists don’t work in industries work in high or low temperatures and so usually don’t have to deal with problems due to workers experiencing excessive heat or cold  stress. However most of us, at some time or other, will be faced with a situation where workers are complaining that an environment is uncomfortable.

The BOHS is holding a Regional Meeting at Ellesmere Port tomorrow on the thermal environment. I’ll be giving  a presentation on how to evaluate thermal comfort. These are the slides I’ll be using.

A version with some explanatory notes is also available here on Slideshare