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Spring is often the most polluted time of year in western Europe. Spring smogs can cause particle pollution to reach the top value of 10 in the UK air quality index, but four to nine is more typical.

However, with the lockdown in place, the increases were less than normal. The air quality index peaked at three over most of England and Wales, while some places in south-east England, Yorkshire and north Wales reached four, the level where health advisory messages are issued.

In March 2014, Paris and many other French cities banned half their cars to control a spring smog. Like Paris, many cities across Europe restrict traffic, reduce speed limits and subsidise public transport during severe smogs. These types of actions have been found to reduce air pollution by around 15%-20% in areas with a lot of traffic. However, during spring smogs, polluted air spreads from country to country, so it is hard for any city to completely control these episodes on its own.

With countries in lockdown, why was air pollution still as high as three or four recently? Chemical analysis of pollution particles conducted by King’ College London showed traffic sources along with gas combustion for power generation by industry and for home heating. Wood burning in London’s homes added to the mixture.

Many particles also included ammonia which comes from agriculture; crops are being planted and fertilised, and manure is being spread on fields over the UK and Europe. It is these agricultural emissions that make spring our most polluted season.

A study of the Sars epidemic in China suggested that infected people were more likely to die if their area had poor-quality air. It is too early to tell whether avoiding this smog will have helped those people suffering from the Covid-19 virus. One big lesson is that air pollution control comes from many sources, and farmers and those with wood fires will need to be part of the fight for clean air.