A new report launched during London Climate Action Week says extreme heat is becoming one of the world's most underestimated threats to economic development, reducing productivity, increasing deaths and placing a disproportionate burden on women working in the informal economy.

The report, Counting the Cost of Heat: The Case for Urgent Solutions for Cities, was published by Heat Resilience Action (HERA).

It examines the economic and health impacts of extreme heat in four cities, including Ahmedabad in India, Bangkok in Thailand, Monterrey in Mexico and Freetown in Sierra Leone, with different climate conditions.

According to the report, extreme heat already reduces annual economic output by between 4% and 8% of a city's Gross Domestic Product (GDP) across the four cities studied and contributes to more than 1,000 deaths each year.

It warns that, without action, the impacts of extreme heat could increase by between three and five times by 2050 as climate change, urbanisation, and ageing populations accelerate.

The report also estimates that women working in the informal sector globally lose about US$57bn in earnings each year due to extreme heat. The losses represent between 4% and 11% of their annual wages.

HERA said around 740 million women worldwide work in the informal sector, where many have limited access to labour protections and social safety nets.

The report found that heat-related mortality accounts for a larger proportion of deaths among women than men, by as much as 20%, because of a combination of biological, social and economic factors that increase women's exposure and reduce their ability to adapt.

It says the economic consequences extend beyond individual workers because women reinvest much of their income into their families.

According to the report, women typically reinvest up to 90% of their earnings in household needs, meaning income losses linked to extreme heat can reduce spending on education, healthcare and nutrition.

In Bangkok, the report estimates that extreme heat reduces women's annual spending on their children by about US$500.

Heat-related productivity losses reduce annual GDP by an average of 4%, rising to as much as 8% during exceptionally hot years in the same city.

In Freetown, the report estimates that extreme heat increases the average household debt-to-income ratio by 3% each year, reducing investment in education and small businesses.

In Monterrey, it is projected that heat-related pre-term births could more than triple over the next 25 years.

Across the four cities studied, annual productivity losses among women range from about 3% in Monterrey to 11% in Bangkok.

The report says women are consistently more likely than men to work in informal employment. In Freetown, it is estimated that 91% of employed women work in the informal sector, compared with 83% of employed men.

Researchers also identified rising night-time temperatures as an important but often overlooked health risk.

The report says hot nights prevent the body from recovering from daytime heat exposure and, together with prolonged heatwaves, account for around 85% of heat-related deaths.

It notes that lower-income households are particularly vulnerable because many homes are built with materials such as corrugated iron and tin, which retain heat.

Despite the growing risks, the report says relatively low-cost adaptation measures could significantly reduce the health and economic impacts of extreme heat.

It identifies interventions including heat response plans, expanding urban green spaces, installing cool roofs, strengthening labour protections and introducing heat-related income insurance.

According to the report, these measures could reduce heat-related deaths in the cities studied by more than 36% by 2050.

It estimates that heat response plans generate economic returns between 12 and 90 times their cost, while cool roofs can reduce indoor temperatures by 2°C to 7°C immediately after installation.

The report also says heat-related income insurance designed for informal workers could reduce women's income losses by more than 40% by 2050.

HERA says adaptation measures should be designed to meet the needs of vulnerable groups because conventional responses often fail to reach informal workers and low-income communities.

Alongside the report, HERA launched an online tool that allows policymakers and researchers to examine the impacts of extreme heat on mortality and economic output in 11,408 cities across 190 countries.

The tool also enables users to compare the costs and benefits of different adaptation measures and assess how heat-related risks could change by 2050.

The report calls for sustainable financing, stronger partnerships across governments and organisations, improved data on the impacts of heat on women, and greater public awareness of heat-related risks.