Air pollution is a serious problem. It kills 40,000 UK citizens every year. There is a 14% chance you'll die earlier if you live in a polluted area, and even if you leave you aren't safe; a heightened risk of premature death stays with you for decades after exposure.
It also has an unfortunate habit of not respecting borders: over one-third of air pollution in the UK is blown in from across the English Channel.
This makes cross-border co-operation crucial. We need common rules and limits to avoid countries undercutting each other to score a short-term competitive advantage, which would come at the price of people's lives. Without these cross-border rules, it would be a free-for-all.
That's why Sadiq Khan and his Parisian counterpart, Mayor Anne Hidalgo, have written to the British and French governments calling on them to sign up to a range of ambitious measures in the National Emissions Ceiling Directive (NECD).
I've been working tirelessly on this very un-sexy sounding but crucial piece of air quality legislation which is currently being hammered out by EU national governments and MEPs in the European Parliament, since I was elected in 2014.
This groundbreaking initiative by Europe's two biggest cities takes us that bit closer to achieving the reforms we need, putting the pressure squarely on the Government. Sadiq's election, and his recognition that this problem needs pan-European solutions is a breath of fresh air from his predecessor, Boris Johnson, who's best response to tackle emissions and air pollution was to "walk on the pavement further away from traffic on busy roads".
Done right, it would set strict and binding targets to reduce toxic emissions across the continent from 2020, 2025 and 2030. It would match the bold lead taken by London and Paris, which have set out hugely ambitious proposals to tackle air pollution, including banning the most polluting vehicles from our streets.
Done the Government's way, the targets will be weaker - leading to an estimated additional 11,000 deaths in the UK by 2030 - and full of loopholes that will make them easier to miss.
The British government has been among the most intransigent against signing up to the current proposals, which MEPs voted through at the end of 2015.The British government has been busy lobbying MEPs and other Member States to lower the overall ambition level of the targets and has joined forces with the powerful agricultural lobby to give farmers a free pass to continue to pollute despite the huge impact agricultural emissions have on air pollution in cities.
The UK is not alone in this - some of the worst-polluting countries including France, Italy, Spain and Poland are playing a similarly dirty game.
Evidence is clear that to tackle the problem effectively, all sectors must contribute to the fight against air pollution, including agriculture.
Ammonia is released by crop-spraying and manure-spreading on big industrial farms. It turns into tiny particles which travel long distances, and was a key contributor to the 2014 smog peak, which was conveniently blamed soley on 'Saharan Dust'.
The problem is so prevalent, that around 4,000 of the predicted 11,000 additional deaths in the UK by 2030 will come from ammonia, as the UK government looks to halve its proposed 2030 reduction targets from 24% to just 11%.
It is European rules and targets that have been crucial in holding this Government to account on air pollution. They were used to drag the Government through the Supreme Court for repeated failure to comply with healthy pollution limits and they are being turned to again because of continued Tory negligence.
Only rules at a European level can truly clean up the air we breathe. For the sake of the British people, the UK government must stop its drive to undermine these new laws, learn from the mistakes of the past, and take the necessary action to address this growing public health crisis.