London pollution is at a record low and as cities around the world grind to a halt due to COVID-19 lockdowns, it has led to a valuable environmental experiment.

In the UK capital, pollution has fallen to its lowest level since records began in the city 20 years ago. The level of nitrogen dioxide (NO2)  caused by diesel fumes has halved.

London's traffic has dropped by more than 40 percent, offices are empty and most planes are grounded.

Tiny particle pollution, which comes from traffic, construction and agriculture, has also fallen but not as far.

Simon Birkett, director of Clean Air in London, says strong easterly winds over the past few weeks have brought particle pollution from the rest of Europe.

"There is no safe level of exposure to particles, they are really very dangerous," Birkett said. "We've been hit by two or three episodes of particles and that has created 'code red' alerts across the southern UK."

Before the lockdown, Euston Road was one of the most polluted areas in London. In January, the nitrogen dioxide level at Euston was 125 percent above the legal limit for protecting human health. Since the lockdown started, the NO2 has dropped and is now at a safe level.

It is estimated air pollution causes 40,000 early deaths per year in the UK.

This month, a Harvard University study found that poor air increases the risk of suffering from severe symptoms of COVID-19.

"What scientists in the U.S. have found is that even small increases in air pollution have resulted in dramatically higher death rates from coronavirus," said Birkett. "That's sadly not surprising, because air pollution causes respiratory problems and it's a respiratory disease."

Under normal conditions, London's nitrogen dioxide levels are double the World Health Organization's recommended limit for human health.

Now people are outside jogging and cycling in clean air. But once the lockdown lifts, pollution is expected to spike again. And the fear is, the joy of exercising in clean air will be quickly forgotten.