Cambodia: The 'burning prisons' fuelled by fast fashion

Chantrea drags an electric fan the size of a large door into the airless chamber where she works every day.

It is her only respite from the heat inside the brick kiln that looks more like a dimly-lit tomb.

"It's like working inside a burning prison," the 47-year-old says as she stacks the dried bricks, which will be moved to a warehouse. "I have asked the owners to provide us with more fans. But they won't because it will cost more money."

The fan she does have slowly clunks as it starts, eventually whirring into action. It barely creates a breeze.

How hot is too hot to work? It is a question researchers have found the answer to here, in Cambodia's brick kilns, where people toil in some of the hottest working conditions in the world, fuelled in part by the scraps of fast fashion.

The BBC spoke to several workers who said they sweat so much through the day that it felt like they were in a hot bath. Fainting is common too, possibly because they become dehydrated. Their names have been changed because they fear reprisals from their employers.

In a first-of-its-kind study, researchers have tried to document how this sustained exposure to extreme heat is affecting workers' health.

Special sensors recorded the core temperature of 30 workers at these kilns over a week and showed that they all had heat stress, or core temperatures of more than 38C. This can cause fatigue, dizziness, nausea and headaches.

A healthy body temperature usually ranges from 36.1C to 37.2C. Body temperature over 38C is symptomatic of a fever. Some workers had core temperatures of 40C, which can lead to heat stroke, resulting in convulsions, eventual loss of consciousness and even death, if not treated early.

One worker told researchers that he had