Formaldehyde, a chemical found in hair straightening products, poses a "serious risk to human health", according to a worrying warning from US health authorities.

In a long-running investigation by authorities into the safety of the chemical, which is used in several industries from textiles to construction, officials warned that the "almost ubiquitous" substance increases the risk of cancer.

Experts have long been concerned about the presence of this substance in chemical hair straightening or treatment treatments.

Listed under the names formaldehyde, formalin or methylene glycol, these substances are used together with keratin, the protein that straightens hair. When heat is applied to the hair at the end of the treatment — from blow dryers or hair stylers — formaldehyde gas is released into the air.

This colorless gas is then inhaled by bystanders, causing, at best, eye and respiratory irritation and, at worst, an increased risk of cancer in the long term.

These risks are compounded if a space such as the living room, toilet or bedroom does not have good ventilation. The new warning, issued by the US Environmental Protection Agency (EPA), states:

"Inhalation of formaldehyde for longer 'chronic' periods can reduce lung function and increase asthma and allergy diseases, as well as cancer."

While the EPA did not specifically name the hair products, the US Food and Drug Administration (FDA) has previously acknowledged the link between hair straightening products and formaldehyde exposure.

The EPA report adds that this substance is also found in some products such as furniture and toys.

The gas can "escape" from these facilities over time, exposing people to the dangers of this chemical. They also acknowledged the risk to workers who may use products containing this chemical without proper protective equipment.