The heat is on for heat stress standards

It is generally accepted that heat stress is a little known and understood issue amongst workers who have to regularly wear protective clothing. More worryingly, the current EN469 standard fails to address the issues arising from heat stress, says Andrew Braimbridge, national account manager from Sioen UK

Heat stress occurs when the body's means of controlling its internal temperature starts to fail. As well as air temperature, factors such as work rate, humidity and clothing worn while working may lead to heat stress. The body reacts to heat by increasing the flow of blood to the skin's surface and by sweating and as sweat evaporates from the body's surface, the body cools down. However, if someone is wearing protective clothing and performing heavy work in hot and humid conditions, heat stress could occur because the evaporation of sweat is restricted and body temperature will rise. The process is then further complicated because the body produces more sweat in an attempt to cool down and so the situation worsens.

The affects of heat stress can range from an inability to concentrate, muscle cramps and heat rash to severe thirst, fainting and heat exhaustion. It can occur in many workplaces such as mines, power plants, boiler rooms, bakeries, catering kitchens and laundries but probably the most familiar and expected work environment is fire fighting.

Fire fighters are more exposed to the risk of heat stress than most other occupations and therefore much can be learnt from the studies carried out in this area of work and the clothing developed to protect them. However, there are still deep inadequacies around the standards written for fire intervention suits specifically in relation to heat stress.

The present EN469 standard for fire intervention suits was published in 2005. It was implemented in the legislation as “Performance requirements for protective clothing for firefighting”.

In 2004, a meeting was organised to agree on the new EN469 text but discussions regarding heat stress revealed that the experts had different opinions.

A series of tests were carried out in the UK, Scandinavia and France that revealed that it was particularly difficult to reproduce the conditions that the fire fighter may experience and therefore difficult to test garments and reach conclusions.

Different wearer trials concluded that it was hard to note a significant difference between the different brands of EN469 fire fighting suits; whatever the brand, the body temperature always increased and therefore heat stress was always high. On the other hand, these tests clearly identified the fittest fire fighter and that some fire fighters were not fit enough! This concluded that one of the most determinant factor for heat stress was metabolic heat production. Consequently, it was concluded that a reduction in work rate would have a larger effect on heat stress than small variations in the heat transfer properties of the protective clothing.

So where are we today?
If an EN469 compliant garment carries the water vapour resistance Z2 mark it must deliver a resistance of less than 30m2 Pa/W.

New and higher performing 'sweating manikins' are now in function in an attempt to reproduce conditions as accurately as possible.

It has become clear that a membrane does not necessarily improve comfort for the wearer when the surrounding temperature is higher than 40°. Consequently, some countries now order fire fighting suits without a waterproof membrane, therefore judging the risk of heat stress as more important than the risk of being wet.

Later this year, the new EN469 standard will be published. Garment manufacturers as well as wearers all look forward to what the new standard will say on comfort testing and how these ongoing discussions can help to improve the comfort of other PPE, such as EN ISO 11612 garments that protect against heat and flame.