Currently the law does not state a maximum workplace temperature, however The Workplace (Health, Safety and Welfare) Regulations 1992 lay down requirements for most aspects of the working environment. Regulation 7 deals specifically with the temperature in indoor workplaces and states that:

‘During working hours, the temperature in all work places inside buildings shall be reasonable.’

The application of what is considered reasonable will vary within different workplaces and the work activities conducted in those spaces. At high temperatures (25°C and above) employees may become drowsy and less aware of dangers. There is also an increased risk of accidents due to slips, trips, falls, poor manual handling, injury from hand tools, etc. Thermal discomfort gives rise to reduced efficiency that can lead to poor decision-making with resultant errors. Work in hot and humid conditions can lead to an increased risk to health because:

•Sweat evaporation is restricted by clothing worn and by the humidity of the environment

•Heat will be produced within the body due to the work rate and, if insufficient heat is lost, deep body temperature will rise

•As deep body temperature rises, the body reacts by increasing the amount of sweat produced, which may lead to dehydration

•Heart rate also increases, which puts additional strain on the body

•If the body is gaining more heat than it can lose, the deep body temperature will continue to rise, eventually reaching a point when the body’s control mechanism itself starts to fail

•The symptoms will worsen the longer the person remains working in the same conditions

Full guidance on this can be found here: 

https://www.abtt.org.uk/product/guidance-note-1-22-working-during-hot-weather/