Employers are being urged to relax dress codes and allow staff to work at cooler times of the day to help them cope with the current heatwave.
The Health and Safety Executive (HSE) said other measures include using blinds or reflective film on workplace windows to shade workers from the sun, and making sure there is plenty of drinking water.
STV meteorologist Sean Batty has said places across the country have recorded new June records and there will be little relief this week.
HSE said the current temperatures for June, and the record-breaking temperatures reached last summer, show that helping workers during extreme heat must be a priority for employers.
Spokeswoman Clare Owen said: “The updated yellow heat-health alert, as well as the extreme heat we experienced in 2022, should serve as a wake-up call for all employers.
“We know that employers are under pressure and we don’t want to add to their burden, but climate change means we’re likely to get hotter summers and that could have a big impact on the workforce of this country.”
TUC general secretary Paul Nowak said: “We’ve all been enjoying the sunny weather after a cold start to the year, but working in sweltering conditions can be unbearable and dangerous, whether it’s in an overheated shop, a baking office, or outdoors in the direct sun.
“Indoor workplaces should be kept cool, with relaxed dress codes and flexible working to make use of the coolest hours of the day.
“Employers must make sure outdoor workers are protected with regular breaks, lots of fluids, plenty of sunscreen and the right protective clothing.”