Labour unionists have called on the government to introduce laws, rather than guidelines, to protect workers in hot weather.

The call comes after a survey carried out by the Hong Kong Federation of Trade Unions (FTU) found that some employers are not following new heat stress at work warning guidelines that were introduced in May. The three-tier warning system advises employers to take necessary measures to prevent workers from getting heatstroke if the warnings are triggered.

But according the survey, carried out by the FTU's Occupational Safety and Health Association earlier this month, half of some 800 respondents' employers had failed to follow the guidelines and arrange rest time for their staff.

Nearly 60 percent of the respondents said they had experienced symptoms similar to heatstroke when working in hot weather.

"Legislation is clearer and more powerful, unlike guidelines - which [employers can] choose to implement or not," Lam Shuk-Yee, the association's chairperson said.

The FTU said employers should also provide sufficient heat stroke prevention equipment to their staff.

The vice-chairman of the association, Wong Ping, said the current heat index system may not be accurate, as the temperature on construction sites can be higher.

The association called on the government to establish heat index monitoring stations to reflect the heat situation in different regions accurately.