Hand washing and water temperature
There is still a common view that hands should be washed in hot water, presumably because it is felt that this is more effective in removing soiling. Studies have shown that water temperature actually plays no significant role in the effectiveness of a skin cleanser.
An alternative view is that hot water is more effective in killing or removing bacteria. Firstly, to kill bacteria the water temperature would have to be so high and the duration of contact so long that severe scalding and potentially permanent skin damage would result. Secondly, again studies have shown that temperature plays no significant role in the removal of bacteria due to hand washing.
Skin and water temperature
In a study of the effect of temperature with surfactants Clarys et al found that even a small increase in the temperature (from 37°C to 40°C) produced a significant increase in trans-epidermal water loss (a measure of the water loss from the skin and hence the deleterious effect on the skin’s barrier properties. This effect was enhanced with certain types of surfactant.
‘…water temperature during washing has an important effect on the onset of irritant contact dermatitis.’ – Berardesca et al, Effects of water temperature on surfactant-induced skin irritation, Contact Dermatitis, 1995
‘In conclusion, our results indicate that temperature is an important variable in the evaluation of the mildness/irritant potential of detergents. … The differences in mildness/irritation properties of the 2 test substances became clearer when using a repetitive immersion protocol with a solution temperature of 40ºC, compared to a solution temperature of 37°C., Clarys P, Manou I, Barel A.O., Influence of temperature on irritation in the hand/forearm immersion test, Contact Dermatitis, 1997
From these, and other studies it would appear that the optimum temperature for skin washing in order not to affect the skin's natural barrier properties is 32ºC and that it should not exceed 38ºC. Excessively hot water will affect the lipids in the lower part of the stratum corneum and so have an adverse effect on the skin's ability to protect against penetration.
Experience shows that very few people are able to judge the correct water temperature. In tests conducted with doctors it was found that most, when asked to set the temperature at 32ºC, actually set a temperature well in excess of 40ºC. This means that to ensure the correct temperature it will be necessary to have this pre-set. However, a temperature of 32-36ºC will be ideal as a breeding ground for legionella.
Thus, the only practicable answer appears to be separate hot and cold feeds to a pre-set thermostatic mixing valve, located as close to the hand wash basin as possible, to ensure water at the correct temperature is provided for the user.
Alternatively, a system for heating the water to the correct, pre-set, temperature, immediately adjacent to the washbasin may prove an alternative approach.
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