Workplace Temperature

Thank you for contacting me about excessive temperatures in the workplace. 
I am proud of the fact the UK now has a world-beating health and safety regime, which provides protection for workers while also ensuring employers are not burdened by excessive regulations. Thanks to the hard work of the Health and Safety Executive (HSE) to simplify regulations and remove unnecessary requirements, businesses have been boosted and workers have never been safer.
As you may be aware, the law does not state a minimum or maximum workplace temperature, though the indoor temperature should normally be at least 16°C (13°C if the work requires rigorous physical effort). The HSE and Local Authorities regulate workplaces to assess compliance with health & safety legislation. They also investigate complaints about working conditions, including excessive high (or low) temperatures. HSE targets its interventions on workplaces in higher risk sectors, such as foundries and bakeries, where high temperatures can be a concern. Inspectors can take enforcement action to improve conditions where they find problems.
A meaningful maximum figure cannot be given due to the high temperatures found in, for example, glass works or foundries. In such environments it is still possible to work safely provided appropriate controls are present. If a significant number of employees are complaining about a high temperature, employers should be carrying out risk assessments and should act on the results. While I understand your concern that there is no legal maximum, I do not believe that setting a universal temperature ceiling across the board is the best way to manage the issue. There is extensive guidance on the HSE website on this topic, including on heat stress, and on the practical steps that employers can take to manage risk. HSE has also simplified this guidance to make it easier to use by small and medium sized enterprises.