The recent pollution episode in London, during which the Mayor Sadiq Khan issued a ‘high alert’ and advised against undertaking strenuous activity outdoors, has reportedly encouraged many residents to search for information about masks to reduce their exposure. However, their efficiency in removing airborne pollutant particles from inhaled air is highly variable and dependent on a range of factors. Although larger particles in the PM10 and PM2.5 fraction may be removed by cheaper masks, many are not effective at removing sub-micron particles, which require an additional filter system rather than simply a fabric mask. Only more expensive models will remove a substantial proportion of these smaller particles, and may prove more uncomfortable for everyday users. In addition, a good fit of the mask to the face is vital in preventing particles effectively by-passing the mask or filter. Advice is generally to use side streets rather than main roads, ensure awareness of high pollution alerts and carry any necessary medication e.g. asthma inhalers, regardless of whether a mask is worn.

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