The law says your employer must maintain a reasonable temperature in your workplace.

Download: Keep your cool! Tackling heat stress at work (leaflet 291)
Date Posted: 23 July 2009
It doesn't specify a maximum
reasonable temperature, but employers are expected to take
reasonable steps to deal with situations where the temperature is
uncomfortably high.
The effects of heat depend
on a variety of factors such as air temperature, heat sources,
humidity, physical exertion, etc. Together these contribute toward
a workers 'thermal comfort'.
Information on Thermal
Comfort can be found on the HSEs website www.hse.gov.uk/temperature/index.htm
Experts say the comfort zone
is normally in the region of 16C to 24C. As the temperature rises
above this zone, heat exhaustion starts. People start to suffer
loss of concentration, there are increases in accidents and loss of
productivity. Symptoms include irritability, dizzyness, headaches,
nausea and fainting.
Sweating also can cause
problems. You can lose as much as two pints of water an hour
through sweating. If fluids and salt are not replaced nausea,
vomiting and muscle cramps can occur. Prolonged sweating can cause
skin rashes and blocked pores (or 'prickly heat').
If your body temperature
gets too high, heat stroke can occur. Research has shown that
people who are not used to high temperatures can only work for an
hour or so at temperatures of 40C before collapsing, even when the
work involves no physical effort.
(* Workplace (Health, Safety
and Welfare) Regulations 1992, Reg. 7).
If you want better health
and safety conditions where you work, make sure you're in
Usdaw.
Keep your cool!
Tackling heat stress at work (Leaflet 291) was correct at date of
publication July 2009.