The Workplace (Health, Safety and Welfare) Regulations 1992 says that as an employer you must maintain a reasonable temperature in the workplace. There is a minimum temperature of 16°c (or 13°C if work involves considerable physical activity) but it does not specify a maximum temperature. However, you are also expected to prevent your workplace being uncomfortably hot (which obviously leads to tiredness and lethargy) and so there should be sufficient thermometers around the workplace so that you can check the temperature. But whatever thermometers read, if most people are complaining of the heat, common sense says that it is too hot and something must be done immediately. However, also remember that how a person responds to heat can also depend on their health, weight and age and so one person complaining of heat may not actually represent a common view. If you have operatives working outside in sunny conditions, you should advise them to wear some form of high factor sun cream and wear suitable clothing, including shirts or T-shirts and hats. Shorts are OK, as long as there are no activities happening where there could be a risk to the person’s legs e.g. using an abrasive wheel to cut something. You should also advise them to drink plenty of water, either bringing some themselves, or providing access to drinking water, if available. However, it would be good if you were also to provide some form of sunscreen for them as if they don’t heed your warnings, or forget to bring any with them and get they sunburnt, they will possibly have to have time off- and if they go so far as to contract heatstroke, this can be very dangerous.