When the average firefighter needs to evaporate about a litre of sweat every hour to regulate their body temperature, understanding heat stress and how to prevent it is useful knowledge for crews.

Fighting fires tests the limits of the human body and can increase a firefighter’s body temperature to over 38 degrees. Firefighting kit is designed to help keep the firefighter’s body within the normal range, protecting health and wellbeing.

Good hydration and heat acclimatisation are important factors to reduce the impact of heat stress on firefighters, but the key component of heat stress control is the selection of Personal Protective Equipment (PPE).

Risks of heat stress

The risks of heat stress on a firefighter can’t be underestimated. Mild heat stress might start with your firefighting crews tiring more quickly, meaning it takes longer to get a fire under control as they are unable to work as quickly. Left untreated, heat stress can lead to poor decision-making as cognitive function deteriorates, and massive pressure on the body’s natural functions, significantly increasing their risk of a heart attack and death.


Insulation is an important part of any firefighter kit, but it stops heat travelling in both directions. While it is essential to keep the heat of the fire away from the human body, it comes with the risk of preventing body heat leaving the suit.

When it’s humid – inside a sweaty firefighting suit, for example – the air in the suit isn’t cool, dry and moving, making it difficult for the vapour to leave the firefighter’s body, reducing the effectiveness of sweating.

Moisture barriers that make a difference

The moisture barrier in firefighting turnout kits help protect against water, chemicals, and viral agents leaching through on to clothes and skin underneath. But they also need to allow moisture out of the kit to minimise the chances of firefighters suffering from steam burns.

All moisture barriers are not created equal. Some suits feature a solid plastic lining, like a bin bag inside your suit. You can imagine how comfortable that’s going to get in the heat of a fire.

A good moisture barrier is designed to let vapour through, but not liquid, making a suit breathable. In this way, the moisture barrier plays an important role in regulating body heat as it allows as much sweat and vapour out as possible.

There are three types of moisture barrier product technology used in firefighters’ protective garments: microporous, monolithic, or bi-component. Each of these barrier technologies has a different level of effectiveness:

• A microporous membrane contains small passages or holes, which allows for air permeability and offers water vapour transfer by air-diffusion.

• A monolithic membrane is a continuous polymer layer without any passages (holes), and, therefore, does not have any air permeability. However, breathable monolithic moisture barriers use hydrophilic polymers which allow water vapour transfer through molecular diffusion instead.

• A bi-component moisture barrier product uses a combination of microporous and monolithic technologies, and allows no air permeability.

Don’t let moisture barriers compromise mobility

When it comes to choosing structural fire suits for your crew, it’s important to consider how easy the suits are to move in, the different requirements of the team and the work they do. Making it easy to move helps to reduce muscular strain and in turn heat stress.

Passing the test

Before you buy firefighting kit for your crew, check the moisture vapour resistance rating (also known as the RET rating), a measure of how breathable the kit is. A lower number is better for this test. Breathability is the key to preventing heat stress. Read more about the tests and standards firefighting kit needs to meet in our blog.

For our full range of products, including moisture vapour resistance ratings, take a look at the catalogue on our website. Or if you want to chat to our team to improve your understanding of heat stress and how to prevent it, give us a call on +44 (0) 1332 341030.

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