A unionist lawmaker said on Friday that the government should consider legislation to ensure that workers are protected from heatstroke if new guidance for employers fails to reduce the number of cases.

Speaking on RTHK's Hong Kong Today programme, Lam Chun-sing questioned whether all employers would follow updated guidelines, which the administration said on Thursday it was planning.

"The guideline is very clear," he told RTHK's Ben Tse. "For example, the government suggests you arrange the work before 10am or after 4pm and you should increase the air flow, have a good ventilation system, and the workers should wear thin and permeable clothing, and to provide sheltered areas for rest and cool drinking water.

"But it's not legislation and only guidelines, only to encourage the employer to follow, so there's a question that whether most of the employers will follow the guidelines."

Official figures show that about 50 people have been treated for heatstroke in the past three years.

Announcing plans for new guidelines on Thursday, Labour minister Chris Sun said the guidance wouldn't just be just about how hot the day is.

He said officials are studying whether to add the Observatory's heat index as one of the reference factors, calling it a scientific and objective indicator of the actual weather situation.

Asked whether employers must follow the guidelines, the minister said bosses are legally responsible for ensuring workers don’t suffer from heat stroke.

Last week, the Observatory put into effect a new "prolonged heat alert", warning of a potential threat to health and urging people to pay attention to their physical condition.

"This summer sees hot nights and very hot weather coming along wave after wave," it said in the announcement.

Temperatures are predicted to reach as high as 35 degrees Celsius in the next few days.