This week, air pollution in London was recorded to be worse than in Beijing, prompting London Mayor Sadiq Khan to tweet: "This is the highest level air quality alert. Everyone, from the most vulnerable to the physically fit, may need to reduce physical exertion."
Though I'm pleased our Mayor is putting the health of Londoners first, it's a worrying reality that people in the city must choose between two unhealthy options: inaction, or breathing in pollution.
Mr Khan has quite rightly warned that this environmental issue is a 'health crisis', but there may be an even more profound impact on human health if experts are increasingly forced to warn against exercise.
Those training for the London Marathon agreed with me - with many tweeting their frustrations, both at Mr Khan's warning, and at the smog preventing them from training.
It's unsurprising that being told not to exercise might ruffle a few feathers.
The capital is, after all, the healthiest city in the UK, with nearly half of all adults having a healthy BMI, far better than the national average of 38 per cent. Londoners are also much more likely to exercise than those living in other British cities, and two of its boroughs are featured in the top ten places with the longest life expectancy.
But it's not just runners who were affected by the smog this week. Children are particularly at risk from air pollution and 433 primary schools and 86 secondary schools are located in places that exceed healthy limits, forcing teachers to monitor air quality for the safety of their students. One school in central London even kept young children inside more this week to protect them from the smog.
Medical evidence shows us that children living with polluted air are more likely to develop asthma, and that the development of their lungs can be stunted for life, resulting in reduced lung capacity. That said, there is an obesity epidemic in the UK, and if our young people can't go outside to play, we risk encouraging inaction.
There is no quick fix for addressing air quality, but staying inside should be a last resort. Greenpeace rightly wants the UK Government to address diesel engines, which are the most polluting of vehicles and switch to cleaner alternatives. They also want to make it safer for to walk and cycle in London.
Now is the perfect time to achieve this goal. Commuters are more able than ever to forgo transport and embrace emissions-free travel, whether it's running to work, riding a Boris Bike, or using one of the Cycle Super Highways. London is culturally ready for a shift towards a low-emissions future.
In the EU, more people die from poor air quality than from road traffic accidents, and the same is true for the UK. In London alone there were over 9,500 deaths related to air pollution, whereas the total number of fatalities on the nation's roads was 1,732.
The UK has learned this lesson before, and it's time to push for progress once again. The Clean Air Act was passed in 1956 after some 4,000 people died in the so-called Great Smog in London in 1952. The act gave authorities wider power to establish smoke control zones and later, in 1968, forced industries burning coal, gas and other fuels to use tall chimneys. Since the 1995 Environment Act, local authorities must now reach air quality targets.
The pollution measured this week must serve as a wakeup call to all of us. It is heartening that the Mayor has committed to renewed action on toxic air, including a daily £10 charge on the worst polluting vehicles and additional anti-pollution measures for schools in the worst affected areas. However, improving air quality is something all Londoners can contribute to, and our health stands to benefit from any progress in more ways than one.
By harnessing London's love of fitness, and encouraging a carbon-neutral school run and journey to work, we might very well be able to beat both inaction, and air pollution.Suggest a correction