More outdoor workers could die if legal obligations to keep them safe in extreme heat stay "vague" and "ambiguous", experts say.

Key points: Some guidelines encourage sending workers home when the temperature reaches 37C

Some guidelines encourage sending workers home when the temperature reaches 37C Calls for a temperature cut-off or regular medical checks

Calls for a temperature cut-off or regular medical checks Workers often feel pressured to stay and considered "weak" if they leave

The decision for businesses to send staff home during heatwaves remains mostly at their own discretion, with only a loose commitment under Australian law to "take reasonable steps" to keep their workers safe.

But, with a heatwave currently sweeping much of the country, there are calls for tighter protections — including a temperature cut-off for work in certain industries.

Calls for action in this area have been ongoing for decades, but experts warn the issue is more important than ever due to climate change.

Workers fear for pay, reputation if they ask to leave work

Manual labor, including working with hot tools and carrying heavy objects, is tough when the mercury reaches up to 40C. ( ABC News: Clare Sibthorpe )

Dr Liz Hanna from the Australian National University said the legal obligations for employers to send their workers home in hot weather varied between jurisdictions and were often poorly enforced.

Some health and safety guidelines — as well as certain construction unions — required outdoor business owners to send their staff home when the temperature reached 37 degrees Celsius.

The CFMEU's ACT branch has this policy in place.

Dr Hanna admitted that the reality of a functioning society meant that not all work could simply stop at a certain temperature, but through her research came up with a suggestion for non-essential work.

Dr Hanna studied outdoor workers from all over Australia to see at what point heat-related illness affected them, and in turn has called for a mandatory graded temperature threshold for certain construction jobs.

She said the "graded scale" would consider heat, humidity, apparent temperature and the physical intensity of work, among other factors.

The severe heatwave that has swept across most of New South Wales for the past three days. ( Supplied: Bureau of Meteorology )

'Life-threatening' and 'fatal'

Dr Naroa Etxebarria from the University of Canberra was alarmed to hear workers often felt inadequately protected to walk off the job without fear of serious repercussions.

Her PhD research focused on the rise in body temperature when exerting oneself in extreme heat, and said no-one working with hot and heavy objects should be outside for long periods during the current heatwave.

"It can definitely be life-threatening and even fatal, especially if you have a certain type of health condition," she said.

She said internal temperature rises had much to do with genetics as well as external factors, so no-one should feel weak if they told their boss they could not work.

"A young, fit and healthy person can be just as at risk … and what we found is that when environmental temperatures hit 40C, core temperatures are often at dangerously high levels."

Dr Etxebarria supported calls for stronger and more detailed laws to improve confidence in workers fearful of speaking out, but doubted the solution was as simple as setting a blanket temperature cut-off.

She suggested more regular medical checks of extremely high-risk workers in heatwaves, particularly of body temperature.

"It's not very practical to do it every day, because you need to ingest a pill and so on," she said.

"However, if the environmental temperature is high and they're exposed to this heat, there are definitely better measures that could be in place."

'They're all pretty happy with being sent home'

Xavier Duffy (right) sends his workers home when the mercury hits 36C. ( ABC News: Clare Sibthorpe )

Owner of a Canberra decking company Xavier Duffy has already implemented a similar system to Dr Hanna's proposal, sending his workers home or giving them inside work when it was hotter than 36C.

While his employers did not get paid for the hours they were clocked off, he said it was a policy that still benefited them, not just his business.

"Obviously it's not ideal [that they lose pay] but if you push them, and they work out in the heat, they're going to get extremely dehydrated and they're going to get heatstroke, which is a really difficult thing to get over," he said.

"They are all pretty happy with being sent home in this heat."

But not all business owners had the same view.

Several tradesman, from tar-layers, to plumbers, to outdoor tilers said they had not been sent home early so far this week when temperatures reached 40C.

The tiler, who did not want to be named, said it was not fair.

"I hate it," he said.

"But I feel like I couldn't say anything because my boss doesn't offer it and the other guys would maybe think I'm a bit soft or they'd have to pick up more work."

'There will be more deaths'

Dr Hanna acknowledged financial concerns for both employers and employees if staff simply clocked off.

She instead suggested a system where tradesmen could start work even earlier than usual on days of extreme heat so they could leave only a couple of hours earlier, instead of missing out on half their shift.

She also noted that productivity declined extremely quickly when heat-related symptoms begin and could cause workers to take several days off work.

Carpenters climb ladders and polish desks in the sun ( ABC News: Clare Sibthorpe )

"While I'm advocating for stronger protections, they become futile if they're not binding or indeed if people find them irrelevant and not practicable," she said.

"But certainly what we're going to find is that if nothing changes, there will be deaths, there will be more people who fall ill or push themselves too hard because of the heat.

"You can do yourself permanent damage, even if you don't keel over onsite."

She said the prevalence of heat-related deaths was poorly documented, as they often did not occur immediately, but that there had certainly been deaths in the ACT related to heat stroke.

Call for stronger laws been around for decades

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The call for stronger laws is not new — a coroner urged the construction industry to set a temperature level for halting heavy outdoor work in extreme heat after the 2013 death of Glenn Newport in southern Queensland.

But Dr Hanna said the need for change was now more urgent than ever.

"This extreme heat that we're suffering now and increasingly so is a relatively new phenomenon," she said.

"We've always had hot days, but they were rare. Why it's become an issue, where we need to do serious planning, is because we're going to have more and more of them.

"Naturally the more we have and the hotter they get, the greater the risk of people succumbing [to the heat]."

'No one-size fits all': WorkSafe

ACT Work Safety Commissioner Greg Jones said a temperature cut-off was not the answer, because there was no "one-size fits all" approach to improving the safety for outside workers.

But he said employers should send workers home or give them indoor jobs when it was not possible to give enough breaks or limit the physical intensity.

"Workers should not be forced or required to work out in extreme heat," he said.

"It's a legal obligation for all employers to provide a safe place for all workers."

However, UnionsACT secretary Alex White agreed with Dr Hanna that this legal obligation lacked detail and was rarely properly enforced.

"If you're working over 37C outside, you should be sent home," he said.

"But at the moment we have very few guidelines from WorkSafe to employers on what they should do … workers are also afraid to exercise their rights.