Heatwave in India – implications for workers

It’s rare that the concerns of ordinary workers makes the news in Europe, but over the past few days there have been reports in the British press about fatalities being caused by a severe heatwave in India.

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(Picture Source; www.commondreams)

According to the Guardian over 2,200 people have died

Andhra Pradesh has been hit the hardest, with 1,636 people dying from the heat over the past month and a half, a government statement said. A further 561 people have died in neighbouring Telangana, said Sada Bhargavi, a state disaster management commissioner.

Environmental conditions have been severe. The Guardian reports

Daytime temperatures hovered between 45C and 47C (113-116 F) in parts of the two states over the weekend, 3-7C (5-12F) above normal, said YK Reddy, a director of the Meteorological Centre in the Telangana state capital of Hyderabad.

The risk from heat stress depends not only on the environmental conditions but also on other factors, particularly

  • workload
  • clothing
  • individual susceptibility

Workers carrying out heavy work for prolonged periods in hot conditions are particularly at risk as they generate significant “internal” heat as well as absorbing it from the environment. Susceptible individuals include the elderly and people who are malnourished. Not surprisingly, then, the majority of the people who have died during the heatwave have been the elderly and manual labourers working outdoors.

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Agricultural workers in India (source Wikipedia)

There are over 3 million construction workers employed in India (the figure is likely to be higher if “informal” workers are included) and many millions more working in agriculture (almost 50% of the workforce). Construction and agriculture are also major industries in other hot countries in the developing world, and there have been reports of numerous heat related illnesses and fatalities linked to manual work in hot conditions in countries such as Qatar, where there has been a boom in construction due to preparations for the 2022 World Cup.

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Construction workers in Qatar (toehk under a Creative Commons Licence via New Internationalist)

The most effective ways to minimise the risks are to prevent exposure or to introduce engineering controls supplemented with work organisation and protective clothing. However engineering controls are impractical in most cases for outdoor workers in the developing world and the so the main way to minimise the risk of workers being adversely affected is to restrict the working time through work:rest regimes or “self pacing” and other administrative /management measures such as providing plenty of cool drinking water. In practice, most employers are unlikely to look favourably on this due to the impact on productivity and profitability. Subsistence farmers are unable to afford the technology available in the developed world (such as air conditioned cabs and automation) that could be applied to reduce their heat exposure and workload and need to work hard for long hours to have a chance of growing enough to survive.

Extreme events like the current heatwave in India are likely to become more frequent in the future due to climate change and it’s not just the developing world that will be affected. The populations of Europe and the United States are also likely to face exposure to heat extremes.

These events present challenges to occupational hygienists. First of all we currently don’t have an adequate method of evaluating the risk in these situations, particularly in the developing countries. The widely used WBGT index has serious limitations and the more complex Predicted Heat Strain Index is far too complex to be used in most situations. So work needs to be done to develop a suitable approach to risk assessment for the developing world. Secondly, given the scale of the problem, there’s a need to find appropriate, effective strategies to reduce and control exposures. Neither are easy tasks. However, some good work has been done on this in countries including India and Abu Dhabi and so the third challenge is persuading employers to adopt the guidance.

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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.

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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.

Heat stress indices

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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

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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.

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* 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