Parents in the UK who leave their car engines running while on the school run could face fines as part of a drive to cut air pollution.
Proposals from the National Institute for Health and Care Excellence (Nice) and Public Health England (PHE) say "no-idling zones" should be brought in outside schools, hospitals and care homes in a bid to protect vulnerable people from fumes.
Westminster City Council is one of those to already have introduced no-idling zones, with officers able to issue motorists with an £80 Penalty Charge Notice if they leave their engines idling when they park up.
According to the council, a car idling for one minute produces enough exhaust emissions containing harmful chemicals to fill 150 balloons.
Nice and PHE say the move would help protect those who are most at risk from air pollution, including children aged 14 and under, people aged 65 and over and those with conditions such as asthma or heart problems.
The guidance says bylaws could be introduced as a way to enforce no idling.
Professor Paul Lincoln, chief executive of UK health forum and chairman of the Nice guideline committee, said: "Air pollution is a major risk to our health and so far, suggested measures have not managed to tackle the problem sufficiently.
"This guidance is based upon the best evidence available. It outlines a range of practical steps that local authorities can take, such as the implementation of no-idling zones, to reduce emissions and protect the public."
The guidance, which is aimed at councils, staff working in transport, employers, health workers and the public, also aims to raise awareness of cutting car journeys through more walking and cycling.
It also suggests ways to promote driving "in a style that minimises emissions by avoiding rapid accelerations and decelerations ... and ensuring the vehicle is correctly maintained".
Other measures include providing charge points for electric vehicles in workplaces and residential areas, and promoting car sharing schemes or car clubs.
The guidance also says that where traffic congestion is contributing to poor air quality, councils should consider incorporating a congestion charging zone within a clean air zone.
Real-time data could tell drivers about optimum driving speeds on roads, while speed bumps could be removed in areas where average speeds are already low.
PHE figures suggest that long-term exposure to particulate air pollution accounts for the equivalent of around 25,000 deaths a year in England.
The health impact of air pollution caused by humans in the UK is estimated to cost between £8.5 billion and £18.6 billion a year, Nice and PHE said.
Professor Paul Cosford, PHE's director of health protection, said: "Many of us can walk or cycle instead of using the car, particularly on short journeys. We can all avoid idling our engines and drive more smoothly to reduce emissions.
"Councils can include low and zero emission strategies in their plans.
"For example, providing charging points for electric vehicles and introducing clean air zones which can include restrictions or charges for certain types of high polluting vehicles.
"Healthcare professionals can advise individuals, particularly those who are most vulnerable, on how to reduce the personal impact of air pollution.
"For example, by reducing strenuous activity when air pollution is high, and by using less polluted routes in towns and cities."
Dr Penny Woods, chief executive of the British Lung Foundation, said: "These guidelines bring into sharp focus the reality that air pollution is one of our most important public health issues. Air pollution is poisonous.
"We welcome the recommendations to introduce no vehicle idling areas. This will help to protect the most vulnerable in our society."
RAC roads policy spokesman Nick Lyes said: "We welcome the principle of no-idling zones, especially outside schools, hospitals and care homes.
"No one should have to suffer dirty air as a result of a driver leaving their engine on unnecessarily. Sadly, many drivers don't realise the harm they are causing by doing this.
"Schools should work closely with local authorities to first encourage parents to switch their engines off. It's right that those that then persist in leaving them on should be subject to a charge."