Maximum heat limits are needed to protect the vulnerable in rest homes and childcare centres, a climate change researcher is urging the government.

As temperatures and New Zealand's elderly population rise, more people will be at risk from heat stress.

Heat already kills more than a dozen New Zealanders a year, and that is expected to get worse as temperatures increase.

NIWA climate scientist Gregor Macara says the number of days which feel uncomfortably hot will rise as global heating pushes up average temperatures and the frequency of hot extremes.

He has already noticed changes in seasonal weather summaries, even in his hometown of chilly Invercargill.

University of Waikato climate change lecturer Luke Harrington says the hottest days of the year have already warmed by more than half a degree Celsius for many New Zealand cities.

A city like Auckland is expected to see the number of days hotter than 29C or 30C rise more with further heating.

Cities with smaller temperature ranges such as Auckland and Hamilton may face higher risks, research by Dr Harrington and professor Dave Frame has shown. Because their temperatures are more even, their hot spells tend to be longer, with less reprieve overnight.

This would not just mean more beach days.

Dr Harrington cites international research showing between one and eight percent of all warm season deaths could be attributed to the weather.

Although New Zealand will not experience the extreme heat seen in places such as Australia, research from climates as varied as Scandinavia and the tropics showed adverse health impacts happened when temperatures soared above what was normal for that area, he said.

What counted as dangerously hot depended on what people are used to and what their built environment is like, with plenty of cooling trees and reliable air-conditioning helping, as well as behavioural adaptations such as "not going out for a run in the middle of the day", Dr Harrington says.

Heat deaths are often recorded under heart disease or other causes, says GP Dermot Coffey, a representative for OraTaiao:

The NZ Climate and Health Council. But epidemiologists could look back on heat waves and work out how many people were tipped over the edge, who would otherwise have stayed alive.

Heat stress increases cardiac stress and the risk of renal failure, causing health impacts long before the point where a person would actually "cook" to death. But heat is often a less obvious killer than a climate-worsened storm event, such as the deadly flooding in Esk Valley in February, he says.

The Ministry of Education says it is currently appointing a working group on early childcare, which may consider introducing maximum temperatures. Photo: RNZ/ Dan Cook.

Most at risk are the elderly, young children, and those on common medications such as heart medicines or anti-depressants. These groups may not be able to regulate their body temperature quite so well, for example by sweating, Dr Coffey says. While people with mental health conditions such as schizophrenia could have a much higher risk.

Although research was still ongoing, Dr Harrington says early indications are that Māori and Pacific Islanders are also more vulnerable.

But he says we can prepare.

In Paris, vulnerable residents can sign up for a door-knocking service, to make sure they were okay when a heat wave hits. In Aotearoa, MetService has started a trial of heat alerts during the summer months.

Dr Harrington wants the government to introduce maximum heat standards for early childhood and aged care facilities, which currently have minimum temperatures but no legal maximum.

If New Zealand keeps building structures which don't keep vulnerable people cool enough, the country is setting itself up for greater health impacts down the line, he says.

Dr Coffey says he "completely agrees" - and believes there should be standards already, in particular for aged care.

There's evidence heat can be an issue even at current temperatures.

The Ministry of Health says it's not aware of a maximum heat limit for aged care facilities, but they are required to have a window and appropriate heating and ventilation.

The Ministry of Education says it's currently appointing a working group on early childcare, which might consider introducing maximum temperatures.

Macara says a major determinant of heat stress will be how much countries can reduce their greenhouse gas emissions by, with the number of riskily hot days projected for the future heavily dependent on the levels of heating emissions.

A recent UN review found the Paris Agreement had brought projected temperatures down considerably, but countries needed to do much more.

-Eloise Gibson/RNZ.