This is found in the Approved Code of Practice which recommends a minimum workplace temperature of 16C or 13C for high labour-intensive work. It does not however recommend a maximum temperature.
The reason that a maximum temperature is not recommended is that there are certain industries which require very high temepratures to operate such as foundaries which would make such recommendations impractical or uneconomic.
The absence of a maximum recommedation does not absolve the employer of any responsibility however, and it would remain liable if illness or injury occured which could be linked to temperature.
As there is no recommendation in the regulations the answer to the question is that the temperature in the workplace should be 'reasonable'. This of course is a difficult test to ratify as it is subjective and opinions may vary from employer to employer, and employee to employee.
A prudent employer would therefore be well advised to carry out a risk assessment as to the possible consequences of high temperatures in the workplace relating not only to Health and Safety considerations (i.e. Pregnant women) but also to the efficiency of the work force, and to monitor what the risk may be. Regular consultation with the work force would also be advisable.
If you would like to discuss any employment issues in the workplace please contact a member of our Employment Team on 01905 721600