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.

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‘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

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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.

I would certainly recommend thermostatic mixer valves on all hand wash basins. The reason is simply that water temperature is an issue when considering the effect on the skin from hand washing. The ideal temperature is between 32 and 35 deg C. Anything over 38 deg C will result in asymptomatic damage to the skin barrier making it more vulnerable to colonisation by transient micro-organisms and damage from chemicals. Try setting the temperature in a manual mixer tap to 35 deg C and then measuring twhat you have. In a short test years ago in Sweden even occupational physicians found it impossible! So for me the only reliable answer is the thermostatic mixer valve.

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Chris I take on board your comments about cost savings but if TMVs are installed then there is an increased cost to maintain them by a suitably competent person as per manufactureres instructions and an increased Legionella risk due delivered hot water below 50 degrees with potential nutrients, dirt and bacteria in the gubbins of the TMV (including the filter, rubber O rings etc)

Originally Posted by: Alan Haynes I believe that the requirement for TMVs is for showers and baths, where the risk of severe scalding can occur. I don't think the requirements apply to hand basins. But signs about 'Caution - Hot Water' might be appropriate.

Is a thermostatic mixing valve required to a wash hand basin in a disabled toilet in a commercial office block environment?

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Postings made by forum users are personal opinions. IOSH is not responsible for the content or accuracy of any of the information contained in forum postings. Please carefully consider any advice you receive.

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.

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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.

For my senario I think your suggestion to fit "Caution-Hot Water"  labels/stickers instead of retrospective TMVs could be a sensible work around.

‘…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

Alan my senario is a wash hand basin in a  disabled toilet in a commercial office block environment and yes I agree with you especially in a health care senario that TMVs should be installed for whole body immersion risk from showers & baths.

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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.

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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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My comments were less to do with the risk of scalding but more with the effect on skin barrier, and thus on the potential for damage to health from washing in hot water. Water has long been identified as a skin irritant by dermatologists and wet work is the most common cause of occupational irritant contact dermatitis. With the current encouragement for frequent hand washing due to Covid-19 there has been an increase in cases of irritant contact dermatitis. So anything that increases the damage that this causes is something that should be avoided. In my experience the cost of fitting temperature control valves is insignificant compared with any increase in skin damage that hot water (not scalding) will cause. Moreover, when fitted it is often found that they reduce the consumption of hot water and hence reduce the cost of heating it. So they can even be cost saving.

First you need to distinguish (to an extent) between existing and new/modified facilities, and particularly for existing facilities the types of disability you are making provision for.......some disabled people will be perfectly capable of running hot and cold water from separate taps, some won't.

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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.