Coronavirus has led society to adopt a new set of habits, one of which is near-universal mask-wearing. Although the disease has retreated somewhat, people still need to wear them in some settings, including all shops and public transport. Official advice is for people to wear a "face covering", but in reality, some have proven more effective than others.Why do some face masks have valves?
The vast array of potential face coverings includes a litany of materials.
Some people have taken to wearing bandanas or surgical masks, while others debut with more sophisticated looking examples which incorporate a valve.
While they may seem a superficial addition, the valves can aid in preventing COVID-19.
They allow the inside of the mask to ventilate, preventing moisture build-up which could make them less effective.
In the long-run, this allows people to use their chosen mask more than others.
But they end up more expensive than other examples, with traditional uses in construction.
Their hi-tech nature and usage in this field makes them poorly adapted for a pandemic, however.
According to the Centre for Disease Control (CDC) in the US, ventilated masks may protect the wearer, but proved a risk for those around them.
As such, the masks make poor measures against an aspect of prevention named "source control".
They said: "The purpose of masks is to keep respiratory droplets from reaching others to aid with source control.
"However, masks with one-way valves or vents allow air to be exhaled through a hole in the material, which can result in expelled respiratory droplets that can reach others."
Holidays: New travel essentials holidaymakers must have - EXPLAINER
Face mask dodgers face huge increase of fines in coronavirus crackdown - INSIGHT
Face mask policy: Tesco, Asda and M&S update shopping rules - ANALYSIS
"This type of mask does not prevent the person wearing the mask from transmitting COVID-19 to others.
"Therefore, CDC does not recommend using masks for source control if they have an exhalation valve or vent."
Source control refers to the ability of a measure to prevent spread to other people in a given population.
The best source control would ultimately prevent someone with COVID-19 from passing on the disease at all.
But the ineffectiveness of masks with a valve does not mean any other covering would prove more effective.
Studies have proven one type is less effective than none at all, the spandex gaiter.
Gaiters wrap around the neck for people to lift over their mouth when they please, which may prove both more fashionable and convenient.
The reason they have emerged as near useless is because they don't restrict air which can carry virus particles when people breathe.