Lead author Dr Hong Chen said: “Air pollution is a societal problem that can be addressed most effectively through enforceable measures that reduce pollutants in the air we breathe every day, not just on days when air pollution is at its highest - writes standard.co.uk.
“Globally, air quality alert programmes represent one of the most common public responses to protect people from air pollution, but the findings of our study show that air quality alert programmes offer inadequate protection for public health.”
The study, in The Lancet Planetary Health, looked at the impact of alerts on 2.6 million people in Toronto, Canada, between 2003 and 2012.
It was the first study to evaluate the effectiveness of alerts. The results were said to be potentially relevant for cities such as London.
Because of inaccurate forecasting, alerts were sometimes not issued when pollution exceeded trigger levels - while alerts were issued on other days when pollution failed to reach warning levels.
The alerts were associated with a 25 per cent reduction in asthma-related A&E admissions but had no impact on reducing admissions for cardiovascular disease, or deaths from cardiovascular or respiratory disease. This is probably because such people already spend a lot of time indoors.
In London, the Mayor has issued seven air quality alerts, the most recent last September. These are published on bus countdown signs, outside Tube stations and on roadside message boards.
Last week the Standard revealed that air quality breached EU legal limits in almost 50 locations across the capital last year. The worst area was Brixton Road in Lambeth.
Simon Birkett, founder of Clean Air in London, said high pollution could cause “terrible suffering” and alerts were “vital”.
He said: “But as this study highlights, the alerts need to be accurate, or at least precautionary, include appropriate health advice and be combined with meaningful action to reduce pollution at its source. Sadiq must up his game, take bolder action and work with other cities to fight air pollution.”
Mr Khan last October introduced the £10 T-charge, which is thought to have discouraged about 1,000 of the most polluting vehicles from entering the congestion charge zone each weekday.
The T-charge is due to be replaced by the ultra-low emissions zone in April 2019, charging cars £12.50 and lorries £100.
A spokesman for the Mayor's Office said: “This new report endorses the Mayor’s approach that, in conjunction with tough measures to improve air quality, issuing air quality alerts is fundamental to protecting public health. Sadiq believes Londoners have a right to know when there are high pollution episodes so those who are most affected can take measures to protect themselves - from changing their journeys, to carrying their inhalers.
"Unlike the previous Mayor, the Mayor will always be honest with Londoners about the state of London’s toxic air and he believes sharing information about high pollution is the right thing to do.
“London's polluted air is a health crisis and the Mayor is doing everything in his power to protect the public.
"Sadiq is delivering hard hitting measures in London - from the Toxicity Charge, to reducing the number of older, dirtier cars in central london and removing polluting buses from busy routes with his Low Emission Bus Zones. He is currently consulting on expanding the Ultra Low Emission Zone up to the North/South Circular Road which would cover 3.8 million Londoners in the most polluted parts of London."
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