A wet face mask is less effective because water restricts airflow and cuts the amount of viral filtering, scientists have warned.

The World Health Organization says dry three-layer masks – the now-common blue surgical masks are the most recognisable of these – worn over the mouth and nose can protect people from catching coronavirus.

But a former senior WHO official has urged users to change their face covering if it gets wet because "moisture makes masks porous", meaning "all types of mask are essentially vulnerable in damp weather".

Professor Tim Spector, who runs the Covid-19 Symptom Study app, warned the Government that the public aren't aware of the risks involved with a wet face mask, saying it would be "useful if clear advice was issued to the public".

Basildon Standard: A wet face mask is less effective, according to expertsA wet face mask is less effective, according to experts

Karol Sikora, a former official in the WHO's cancer programme, told The Times: "Moisture makes masks porous and because of this all types of mask are essentially vulnerable in damp weather.

"I think the public generally lacks any knowledge or guidance on this issue. They should be given clear advice by the authorities, particularly given the current spell of heavy rain.

"It is important that we know to change our masks if they become damp at all - higher-priced masks do not have different rules."

The warning also applies to masks that have become wet due to moisture from breath.

The Nursing Times says these also suffer from an "increased resistance to airflow" and are therefore "less efficient at filtering bacteria and has increased venting".

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The WHO backs up his warning, stating online that all masks should be changed if "wet or visibly soiled".

"A wet mask should not be worn for an extended period of time," the body says.

"Replace masks as soon as they become damp with a new clean, dry mask."

Other scientists added their voices to calls for the Government to make the risks of wearing a wet mask clear to the public.

Professor Tim Spector, who runs the Covid-19 Symptom Study app, warned the Government that the public aren't aware of the risks involved with a wet face mask.

"It would now be useful if clear advice was issued to the public," he said.

"Masks need to be changed regularly and this is particularly important to understand in damp and wet weather."

Guidance published by the Department for Health and Social Care tells people to "change the face covering if it becomes damp or if you've touched it".