A new study highlights the need for widespread use of better face masks and the importance of good ventilation to mitigate the spread of Covid-19 indoors. There is a very serious difference in the effectiveness of different masks when it comes to controlling aerosols. Aerosols dispersed by infected people are a source of transmission of the SARS-CoV-2 virus that causes Covid-19, even outside the two-meter social distancing zone widely recommended by public health officials. Common masks, primarily due to problems with fit, filter about 10 per cent of exhaled aerosol droplets. By contrast, higher-quality, more expensive N95 and KN95 masks filtered more than 50 per cent of the exhaled aerosols that can accumulate indoors and spread the Covid-19 virus when inhaled by other people.