Air quality index peaks at three across England and Wales, but wood fires and farming continue to cause pollution

We think of spring as the time of blossom and fresh new green growth, but it is often the most polluted time of year in western Europe. Last week, as winds turned easterly, particle pollution once again spread across 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.

With the lockdown in place, the increases were less than normal. The air quality index peaked at three over most of England and Wales. A few places in south-east England, Yorkshire and north Wales reached four, the level where health advisory messages are issued. After three days, a welcome change of wind direction at the weekend pushed the polluted air southwards.

In March 2014, Paris and many other French cities banned half their cars to control a spring smog. Most of the UK was also affected, but actions here are limited to advising elderly and vulnerable people to refrain from outdoor exercise. A second smog struck that April and the then prime minister David Cameron was heavily criticised by opposition MPs and European officials when he tweeted that the bad air quality in London was harmless Saharan dust.

It was not. Later, Public Health England estimated these two smogs led to around 600 early deaths in England and Wales. Spikes were also seen in people arriving at hospital emergency departments in London and Paris, especially in young people with asthma.

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.

Quick guide What to do if you have coronavirus symptoms in the UK Show Hide Symptoms are defined by the NHS as either: a high temperature - you feel hot to touch on your chest or back

a new continuous cough - this means you've started coughing repeatedly NHS advice is that anyone with symptoms should stay at home for at least 7 days.

If you live with other people, they should stay at home for at least 14 days, to avoid spreading the infection outside the home. After 14 days, anyone you live with who does not have symptoms can return to their normal routine. But, if anyone in your home gets symptoms, they should stay at home for 7 days from the day their symptoms start. Even if it means they're at home for longer than 14 days. If you live with someone who is 70 or over, has a long-term condition, is pregnant or has a weakened immune system, try to find somewhere else for them to stay for 14 days. If you have to stay at home together, try to keep away from each other as much as possible. After 7 days, if you no longer have a high temperature you can return to your normal routine. If you still have a high temperature, stay at home until your temperature returns to normal. If you still have a cough after 7 days, but your temperature is normal, you do not need to continue staying at home. A cough can last for several weeks after the infection has gone.

Staying at home means you should: not go to work, school or public areas

not use public transport or taxis

not have visitors, such as friends and family, in your home

not go out to buy food or collect medicine – order them by phone or online, or ask someone else to drop them off at your home You can use your garden, if you have one. You can also leave the house to exercise – but stay at least 2 metres away from other people. If you have symptoms of coronavirus, use the NHS 111 coronavirus service to find out what to do. Source: NHS England on 23 March 2020

Globally, Beijing has been the venue for the most successful smog control schemes. These were devised for the 2008 Olympics, but have since been used for other high-profile events. Industry around Beijing was curtailed and traffic cut by half for the Asia Pacific Economic Conference in 2014, and the 2015 parade to mark 70 years since the end of the second world war.

Suddenly, the people of Beijing could see the true colour of the sky without the customary haze. It was nicknamed “APEC blue” and later “parade blue”. These controls took place over an area of around 500,000 km2, covering a population of nearly 300 million people.

I have often wondered what would happen if we did the same in Europe, but I thought I would never find out. In 2017 a team of French scientists used a computer model to predict what might happen if all of Europe shut down for a day to control a springtime smog. Particle pollution would fall by about 20%-40% in major cities. Last week we saw Beijing-scale measures implemented across Europe for the first time.

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

Many particles also included ammonia in their make-up. This 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. In Dublin the in 1980s, it was a deterioration of survival rates inside hospitals that prompted the city to ban the most polluting types of coal. 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.