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As Britain sweltered through the country’s hottest day on record last month, supermarket delivery vans wove around the streets as usual delivering shopping to people’s homes. But while the vans have technology to keep the food and drink chilled in the back, a surprising number don’t provide the same service to the humans in the front.

Sainsbury’s, for example, confirmed to me its vans don’t have air conditioning in the cabs. The supermarket chain said it gave drivers more frequent breaks, access to cold drinks and a relaxed dress code during the heatwave.

Tesco’s older vans don’t have air conditioning either, though its new electric ones do. The company said drivers without air conditioning were “able to stay safe and comfortable with ventilation, regular breaks and plenty of water”. Ocado told me three-quarters of its fleet has air conditioning and that will soon rise to 90 per cent. Waitrose, meanwhile, has air conditioning in all its vans.

Air conditioning is not normally fitted as standard and costs about an extra £1,000 per van, according to Justin Laney, Waitrose’s fleet manager. “Anybody doing a job like that, it’s quite hard and quite manual, you’re sometimes required to lift heavy loads up staircases, it’s quite sensible you’d give the person the best possible comfort when in the van,” he said.

High temperatures have always been a hazard for outdoor workers such as builders in hot regions like the Middle East. But as the planet warms and heatwaves become more frequent, the range of countries, workers and employers affected is set to widen. The chance of the maximum daily temperature exceeding 35C somewhere in the UK has already increased from once every 15 years in the mid-20th century to once every five years today, according to one Met Office study.

As a result, more employers have to reckon with the consequences — both for health and safety and for productivity. Albert Heijn, the Dutch supermarket chain, suspended home deliveries in the Netherlands altogether at the height of Europe’s heatwave, saying it wasn’t “responsible to let our delivery workers work in these weather conditions”.

The most obvious occupational danger is overheating. Heat stress can cause muscle cramps, fainting and exhaustion. Heat stroke can also kill. Older people are less able to cope with high temperatures — a particular concern where the working population is ageing, such as in Europe. Last month, a 60-year-old street sweeper in Madrid died of heat stroke after collapsing at work.

Workplace accidents happen more frequently in hot weather, too, perhaps because hands get sweaty or concentration levels fall. A study published last year by UCLA compared records from more than 11mn California workers’ compensation claims with local weather data. On days with temperatures above 90F (about 32C), workers had a 6 to 9 per cent higher risk of injury. When the temperature topped 100F, the risk was 10 to 15 per cent higher.

Some health risks are more long-term. In Central America and other regions, abnormally high numbers of young workers in hot conditions such as on sugar cane plantations have been dying of chronic kidney disease in recent decades. Researchers don’t know for sure what causes this, but many believe heat exposure and dehydration are important factors, possibly combined with agrochemicals.

Then there is productivity. The International Labour Organization says that at 33 to 34C, a worker operating at moderate intensity loses 50 per cent of work capacity. It has predicted that by 2030, the equivalent of more than 2 per cent of total working hours worldwide will be lost every year, because it is too hot to work or people work more slowly. In south Asia and west Africa, that figure might reach 5 per cent.

How will employers adapt? Some measures are straightforward. Supermarket chains in the UK can put air conditioning in new vans, for example. Tesco plans to make its whole home delivery fleet electric and air-conditioned by 2028. Sainsbury’s said it would “revisit” its measures to keep workers cool and “make any necessary changes should the need arise”.

Other problems are harder. One study of US farmworkers found increases in rest time and the provision of air-conditioned rest areas would be effective but could also “affect farm productivity, farmworker earnings and/or labour costs”.

Unions in the UK and EU are pushing for laws on maximum working temperatures. Only a few European countries now have legal limits, which range from 28C to 36C. Yet, it is workers in precarious jobs with little union presence who are at the greatest risk.

In the regions most exposed to rising temperatures, informal employment and weak safety nets are common. Even in rich countries, studies show that agricultural workers employed on piece rates are more likely to suffer from heat stress than those paid by the hour, since their income depends on how fast they work. Piece rates are also common in the gig economy.

It is often said that the pandemic split the world of work into those who could work at home and those who couldn’t, but in truth the virus mostly exposed cracks of inequality that were already there. The warming planet is likely to do the same.

sarah.oconnor@ft.com