The heat-moulding process mentioned in the patent can no longer be done in the UK as cup-moulding is outsourced (there is a single moulding machine preserved as a museum piece at De Montfort University in Leicester).

However, moulded cups are not the only similarity between bras and masks.

To be effective, a mask must fit closely to the face, which is usually achieved with a flat piece of wire over the bridge of the nose. Bra makers are, of course, accustomed to working with textiles, wires and elastics to form a snug fit.

“Fit is key for masks,” said Dave Morris, a technical bra developer. “Faces are a more difficult shape than breasts because of the nose but bra makers are well-used to forming a 3D shape to the body.”

The Belgian company Van De Velde, which owns the lingerie brand Rigby & Peller, is making masks for hospitals in Belgium. The company has also released open-source instructions to enable people to make their own barrier masks.

Bra-making methods have been applied to masks before. In 2009 a doctor who helped evacuate and treat children from Chernobyl won the IG Nobel Prize in Public Health for inventing the Emergency Bra, which transforms into two masks.

Kenyon’s masks, which she is distributing for free, are washable enabling frontline workers to re-use them. Kenyon, who comes from a line of Lancashire clog-makers, is now trying to scale up production.

“I’m not a mask expert,” she says. “Ideally these masks would be made by experts and thoroughly tested but this is a fast-moving situation and there does seem to be a clear need for masks for frontline workers.
 
“The studies I have read suggest something is better than nothing. I hope home sewers will join the effort as they are doing in America.”

Scientists from the University of Cambridge tested the efficacy of homemade masks after the H1N1 flu pandemic. They concluded that homemade masks should only be considered as a last resort to prevent droplet transmission from infected individuals in a pandemic but would be better than no protection.

There are three main types of mask: surgical mask, disposable respirator and reusable respirator. A surgical mask is a physical barrier primarily designed to protect the patient from droplets of saliva when the clinician exhales. Surgical masks do not provide a tight seal to the wearer’s face and smaller particles may penetrate the mask’s layers.

Respirators provide a higher level of protection from viruses. They have filters to protect the wearer from tiny airborne particles and some have exhalation valves to reduce condensation. The US-standard N95 mask filters 95 per cent of airborne particles that are 0.3 microns in diameter or larger whilst the equivalent European-standard FFP2 mask filters 94 per cent and the FFP3 filters 99%.

Reusable respirators with replaceable filters are the ones that make the wearer look like a big-eyed bug. They are commonly used to protect workers from industrial gases or vapours. Respirators and surgical masks only provide full protection when worn and handled correctly.

Large manufacturers are currently grappling with the challenge of producing the FFP2 and FFP3 respirator masks desperately needed by the NHS.