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How much do we sweat working in the heat? Fluid and sodium losses explained.

Industrial Worker at the factory welding closeup

The average outdoor blue collar worker will sweat around four or five litres in a ten hour shift with losses of eight to 10 litres common.

Extreme conditions such as high temperatures and wearing full body clothing and PPE can even result in sweat rates of up to three litres an hour.

The figures come from two studies into blue collar workers, one on Australian underground miners and the other on outdoor blue collar workers, which focused on sodium losses in sweat.

Even mild dehydration – classified as a fluid loss of between 1-4 per cent of body mass – has significant OHS implications. It impairs work performance and cognitive function, including decision making and visual motor tracking abilities.

With the average lean and healthy 70kg male body containing around 60 per cent (42 litres) of water, the four or five litres lost in a 10 hour shift represents a potentially deadly fluid loss greater than 10 per cent of body mass.

It is critical there are OHS strategies to ensure this fluid – and the sodium within it – is replaced.

In the Australian mining study which was conducted on 39 workers over 39 shifts, the average worker drank 6.48 litres (0.8 litres per hour) with a range of 2.40 to 12.50 litres. These figures do not account for the moisture content in food, which can contribute up to 20 per cent of our daily fluid intake.

The study compared the hydration (measuring specific gravity) of these workers over their shift and determined that most workers ended their shift in a similar state of hydration as they started – which was often already in a state of dehydration.

They concluded that average sweat rates were not be significantly different from average fluid consumption rates, meaning that some workers lost over 10 litres of fluid during their shift.

However the study – citing comparison studies – concluded that because workers’ fluid intake was being monitored, they probably drank more fluids than they otherwise would have.

It states that workplace hydration education is vital to maintaining hydration levels and recommends programmed drinking rather than responding to the sensation of thirst.

Further Australian mining dehydration research found 58 per cent of workers were dehydrated both before and after their shift, some severely, reinforcing the need for strong workplace hydration systems.

What Influences Fluid Losses?

Even without any physical activity, most people lose around two to four litres of fluid each day (three to six per cent of body mass) through urination, faeces, in breathing or through the skin.

Physical activity substantially increases fluid loss, while diet, heat acclimatisation and fitness status, metabolic rate, body mass, size and environmental conditions such as temperature, humidity, wind, altitude, solar load and the amount and type of clothing worn all have an influence. Using environmental monitoring tools will help determine dehydration risks.

For those looking for broad fluid intake guidelines in hot and humid conditions, consuming 250-300ml every 15-20mins is recommended, however not necessarily appropriate for individuals.

Sodium Losses and Replacement:

The fluid and sodium loss study on outdoor blue collar workers states that the need for sodium replacement is often overlooked.

While there were large variables in the amount of sodium the 29 participants lost during the research, they say sodium (Na) loss from sweating averaged 4.8 to 6g of sodium per day and “could exceed 10g per day, equivalent to 25g of salt (NaCl)”.

The study stresses that it is critical this lost sodium is replaced, however cautions against cordials or “sports drinks” with high sugar content.

They cite fluid consumption rates of up to 10 litres as common in hot conditions and state that with these drinks containing up to 10% sugar, the daily sugar intake – if these were the only fluids consumed – would be a dangerously unhealthy one kilogram!

The study recommends that “An ideal fluid replacement beverage for industrial use should have significant sodium content with minimum carbohydrate.”