The Reader: Air Filter Trials in London Schools Are a Big ...

The challenge is huge. Last month, the World Health Organisation (WHO) advised that air quality guidelines for ambient or outdoor air are considered applicable for indoor exposure as well. This means air pollution needs to be reduced, inside and out, by up to 50 per cent at hundreds of schools in London.

Parents and teachers can start now by asking whether their school has air conditioning, mechanical ventilation and/or, crucially, air filtration.

Where WHO guidelines are exceeded, parents could ask schools to install air filters that comply fully with the latest international standards for minimum air filtration efficiency (e.g. BS EN 16798-3:2017), particles (BS CEN ISO 16890-1:2016) and gases (BS CEN ISO 10121-2:2013). Standalone air filters offer an alternative to mechanical ventilation.

This approach works. The City of London installed air filtration in a school nursery last year and measured a real reduction in concentrations. Better still, they brought annual average concentrations of nitrogen dioxide (NO2) well within the WHO guideline inside and out.

It is reassuring that the Mayor is taking action - but others can too.Simon Birkett, Founder and director, Clean Air in London Dear Simon No pun intended but the shocking state of London's air can often take the breath away.

What's even worse is when our obsession with motor travel trumps the health of our children.

This was illustrated last week when council officials in Waltham Forest, seeking to discourage parents from leaving their car engines idling while dropping children at the school gates, suffered intimidation from motorists who thought it acceptable to stop in the middle of the road as they discharged their passengers.

Your campaigning on toxic air is to be commended, as is Sadiq Khan's willingness to take action. His audit will likely find many more than five nurseries in need of help.

Retro-fitting air filtration systems sounds like an expensive and potentially futile exercise. It may help a small number of children for part of their day but, come the school bell, they step out from their pollution-free bubble.

Yes, parents can demand action on "cleaner" buildings - but first they should think twice before revving up their car engines. Let's tackle the cause, not the effect.Ross Tiffany Jenkins's article ["Great art should stay in museums, not be returned to where it was made ," Comment, November made for difficult reading. Ms Jenkins's peak ignorance comes with "Athenians - not Greeks - built the Parthenon", contesting the notion of "Greekness" that ties together the ancient past and the people of the present. Does she imply that invasions (such as four centuries of Ottoman rule that failed to wipe out Hellenic history, language and culture) have rid the Greek people of their claim to their past?Dr Sophia Tsoka King's College London TiffanyJenkins confirmed that she had never been to a museum in Nigeria or similar countries before. If she had, she wouldn't suggest there was no other place suitable for certain artworks. Maybe she would have different concerns. Some of us like to visit new places and their museums in the hope that we can learn more about that location in a profound way.

Loretta Okpokiri