An epidemiologist has revealed the common mistake people make when it comes to wearing a mask and has highlighted why it is a problem with an experiment.

Infection preventionist Sarah Milligan, from Texas, shared photos on Twitter showing how germs are spread if people move their masks and don't wear them properly.

"Other than being completely ineffective at preventing transmission, this is why we want you to wear your masks properly at all times," she wrote on Twitter.

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In one photo, ultra-violet light shows how a person could potentially spread germs from their neck onto the mask if they move it up and down.

The other shows how germs can stick to people's noses if they don't wear their mask correctly and cover their nose.

Milligan advised people if they need to take off their mask, they need to do so carefully without it touching another part of their face.

"Keeping it on will reduce the risk of self-contamination, but if it's too uncomfortable just remove it completely and make sure to perform hand hygiene before and after," she said.

Associate Professor Ben Mullins at Curtin University's School of Public Health, who is an aerosol deposition expert, previously told Yahoo News Australia that wearing a mask for too long can become a hazard while the virus is present.

"Wearing them for too long, as they're supposed to be disposable, they could actually become a vector for disease if someone is exposed to infected patients," he explained.

A high-level scientific NZ review has explored the vexed issue of face masks – finding there's not enough evidence to recommend people wear them to protect against Covid-19.

The review, commissioned by the Ministry of Health's chief science adviser Dr Ian Town, explored a range of approaches that countries have taken.

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It found there was evidence of potential benefits - but also potential harm - around masks and that the science wasn't conclusive.

That's despite a top epidemiologist urging New Zealand to adopt "mass masking" when it drops down to alert level 2.

Covid19.govt.nz: The Government's official Covid-19 advisory website

Otago University epidemiologist Professor Michael Baker earlier told the Herald overseas studies were increasingly showing the value of wearing masks.

One suggested that a non-fitted surgical mask could block up to 100 per cent of droplets containing coronavirus, while another, focused on cotton mask use on infected patients, found viral spread from a cough was cut down by 96 per cent.

"I don't know about you, but I'd feel much more confident in a bus if everyone else had a mask on - that seems like a very small sacrifice to make," Baker said.

Nonetheless, the benefits of "mass masking" still remain debated among public health experts around the world – with some citing worries about them fuelling false confidence, failing to prevent spread, or people simply not wearing them properly.