The law in England has just changed which means it is now mandatory to wear a mask while out shopping.
There are a few exemptions - such as children under 11 and people with some disabilities - but others found without a mask on risk a £100 fine.
The idea is to reduce transmission while letting people shop with more confidence.
Health Secretary Matt Hancock said: "We want to give people more confidence to shop safely, and enhance protections for those who work in shops.
"Both of these can be done by the use of face coverings."
Except, doctors and scientists agree that face masks with valves are more or less useless when it comes to stopping people spreading the disease to others.
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Masks with valves to work well in one way - they can be excellent at purifying the air you breath in.
The point of the valve is that it allows you to breathe out easily, letting moisture escape enhancing comfort and giving disposable ones longer lifespans.
When you breathe in, the valve closes, meaning air is forced through the mask's filters.
But letting moisture and air escape easily is terrible news for stopping the spread of the disease.
Doctor Kai Singbartl, chair of the infection prevention and control at the Mayo Clinic in Arizona, said: "It defeats the purpose.
"They are unfiltered, those valves are the path of least resistance so to speak, it's easier to exhale and get rid of the heat and moisture."
Marisa Glucoft, director of infection prevention Children’s Hospital Los Angeles, said: "When you wear a mask with a valve, people around you are not protected because the valve lets all of your breath into the air.
“From that perspective, it’s almost like you’re not wearing a mask at all.”
In fact, it can even potentially mean you push your breath - and any virus it might be carrying - further than not wearing one at all.
That's because all the air you breathe out is forced through the valve holes, which are normally far smaller than someone's mouth or nose is.
According to a study by the University of Edinburgh respirator masks provide protection to the wearer, but "the valves on these masks that made breathing easier could potentially allow infectious air to spread considerable distances in front".
By contrast, everything else they tested worked well at stopping breath from being pushed in front of you - even hand made ones.
So while masks with valves will comply with the letter of the new law, they are very much against its spirit.
