doesnt need to realistically be one if people don't take the piss. some of the fishnet shit in america being a prime example of people being assholes for the sake of being assholes. as long as it provides some impedance to outbound air flow, it will do the trick. you could use a pair of tights over your head and achieve the same thing.
it's not there to filter the lurgy etc as people are arguing/assuming, its simply the expulsion radius, which is meant to be used in conjunction with social distancing to reduce potential radius of the viral load blasted out in one cough, breath, sneeze, talk.
facemasks are a pointless argument, the real argument is what things are being chosen to stay open and what is being closed. current track and trace data is showing the major infection rate at this point isn't shops, gyms etc but pubs and parties in close spaces. hopefully the government is paying attention to that data and only closing things that need closing.