Climate and Environment

Air temperature and humidity vary greatly depending on the region and season, even across Europe. Warm and/or humid climates raise body temperature and the normal response to this is sweating, which leads to cooling through evaporation. This influences water losses and thus, requirements.

• Elevated environmental temperature
• Air movement
• Humidity
• Intensity and duration of physical activity
• Clothing

Average sweat losses during physical activity are 1-2 litres per hour but can be as low as <0.5 litres per hour, or even as high as >3 litres per hour².

Certain types of workers carry out their normal duties in hot environments and are, therefore, at risk of heat stress. Examples are industrial workers, building labourers, road workers and farmers.

In temperatures of 25-40°C (77-104°F), sweat losses can reach 0.5-1 litres per hour, resulting in fluid requirements of up to 12 litres per day^3,4. However, it is not just water that is lost during heat exposure, this is because sweat contains body salt. Research suggests that salt losses over a typical working shift could be as high as 10-15g, potentially increasing the risk of hyponatraemia (low blood sodium) if the salt is not replaced via foods and fluids. Left untreated, hyponatraemia can have serious health consequences since sodium is vital for blood pressure regulation.

Several studies have found that a high proportion of industrial workers are inadequately hydrated at the start of their shifts^{4, 5} leading to concerns about their health and wellbeing. Experts have called for workers under heat stress to have access to education programmes that aim to promote good hydration throughout working shifts^{4, 5}.

Key points for healthcare staff delivering these types of programs are:

• Recognise that workers exposed to chronic heat stress will lose both water and salt through increased sweating.
• Water needs will be much higher than recommended by the AIs.
• Individual workers vary in their average sweat rate and how much sodium they lose in sweat. This will affect individual fluid and salt requirements.
• In certain working environments, workers may have limited access to fluids or palatable water, leading to difficulties accessing sufficient fluids for health⁴.
• Workers will benefit from education about when and how to replace lost fluids and salt, including promoting access to a range of different beverages.

The effect of cold temperatures on water needs is often underestimated. Although sweating may be expected to be minimal when, wearing heavy or impermeable clothing whilst exercising in the cold, such activity can lead to unexpected sweat losses⁶.

 In cold and dry environments, increased water loss occurs through breathing, with the impact becoming significant for those exposed to such extreme conditions for several hours per day. If the body becomes cold, cold-induced diuresis may lead to an increased production of urine with a low urine specific gravity (USG ~1.009 g/ml)¹. It should be noted that, under such conditions, a low USG could be misinterpreted as indicating euhydration (adequate hydration).

Water losses increase at high altitude because the air becomes drier. This stimulates hyperventilation, which tends to elevate respiratory water losses. Furthermore, exposure to the thinner air of high altitudes lowers blood oxygen levels, leading to hypoxia-induced diuresis^{7, 8}, often occurring within a few days of arriving at higher altitudes. All of this means that workers or climbers at high altitude need significantly more water than those living closer to sea level.

Research shows that people climbing at heights of 1600m to 4393m have an average water turnover of 7.1±1.1 L (95±18 ml/kg) per day, which is substantially higher than in sedentary adults⁸. While, there is no consensus on the exact percentage increase in water requirements at altitude, there are useful assessment tools for identifying dehydration in those working or climbing in these environments, e.g. short-term weight loss, urine colour, urine specific gravity⁷.

• Remember that altitude affects your water needs and plan for regular access to fluid throughout the day (and night if remaining at high altitude to sleep or work).
• Remain aware of the possible consequences of dehydration for yourself and others, e.g. reduced mental performance, which could affect safety and decision-making.
• Learn to recognise the early signs of dehydration, e.g. reduced urine output, headache, thirst.
• There is often limited access to water at altitude (due to dry or frozen conditions). This means that water, or sufficient fuel to heat snow/ice, will need to be carried⁸, with implications for load carrying and rucksack space.
• Increased water losses may be misjudged, especially by novice climbers, because air temperature decreases with increased elevation and dehydration could be neglected^7,8.
• In long distance flights, cabin air has a low humidity, which can cause general dehydration⁹.

In conclusion, temperature, humidity and altitude can increase water needs compared with general recommendations. Healthcare professionals, such as physicians and dietitians, should be aware that climate and environment can influence hydration status and should account for this when giving advice. Furthermore, individual variations in sweat losses make general recommendations difficult to implement, suggesting a need for individual hydration assessments and tailored programmes for those living, working or exercising under extreme conditions.