Sixteen-month-old Madalena grins and throws her flashing ball across the consulting room, oblivious to the conversation going on around her.

Her mother, Patricia Correia, is explaining to the doctor that the family is thinking of returning to Portugal after a series of “wheezing episodes” that have forced the toddler to be rushed to A&E five times in the past six months. Once she was so ill, she was kept in for four days.

“It is very, very frightening because you can see her struggling, starting to breathe really fast,” Correia tells the consultant. “And I know her oxygen saturation level is dropping because I am a nurse. It is so scary.”

Correia fears her daughter is at the sharp end of the UK’s air pollution crisis, which contributes to about 40,000 deaths a year and has profound long-term health implications for millions more.

The family, who arrived in the UK in 2016, is considering a return to Correia’s native Portugal where the air will be cleaner and Madalena may recover. “We came here because in Portugal there are not many jobs for nurses and we thought it would be a good life, but now? It is not easy because I will have to quit nursing but her life … her life is precious.”

The consultation at the Royal London hospital is being carried out by Prof Jonathan Grigg, one of the UK’s leading specialists in respiratory illnesses in children.

In the two hours the Guardian sits in on the session, there is a regular flow of parents and children into his room.

The stories follow a pattern: babies and toddlers with severe wheezing episodes, older children with sometimes crippling asthma. Anxious parents are offered information and reassurance, a range of medicines are talked through and future appointments booked.

One nine-year-old boy has had such a severe episode it triggered a heart attack and he must now use a wheelchair. A 20-month-old has been in and out of A&E in the past few months and his father wants to know if he is in danger.

Last year, figures from the Office for National Statistics revealed a record number of people were dying of asthma, with experts warning that air pollution and a lack of basic care could be to blame.

As the consultations come to an end, Grigg, who set up the Doctors Against Diesel campaign and is a leading member of Royal College of Paediatrics and Child Health, said the patients’ stories were all too familiar.

And although, he said, it was impossible to ascribe air pollution as the sole cause in any individual case, the UK’s filthy air “is very severe and is a major driver of adverse respiratory effects”.

“The WHO said recently that air pollution is the new tobacco and I think, in terms of its impact on health, that is how we should think of it.”

Grigg says awareness of air pollution has grown significantly in recent years, with patients often ahead of the medical profession in terms of understanding. A growing number of people were asking him whether they should try to move house or school – or, as with Correia, move country – to avoid the impact of the UK’s air pollution.

“I am wary of advising people because, hand on heart, I can’t say their child will get better if they move and often people do not have that option. Rather, I advise them to come together with other parents and ask the school or the council what can be done, why there are so many cars on the roads outside their schools or homes … collective action seems better than trying to deal with this as individuals.”

The scale of the crisis has become more evident over recent months – especially its impact on children. Reports have emphasised the links between illegally poisonous air and heart disease, dementia, reduced cognitive ability and asthma deaths.

Schools are becoming increasingly vocal, demanding that parents and children walk rather than use cars to get to and from school. Others are installing air purifiers in the classroom after growing concern around levels of indoor air pollution.

Some regional politicians – Sadiq Khan in London and Andy Burnham in Manchester – are beginning to take action, although campaigners say the plans do not go far enough, fast enough, and many medics and environmentalists are increasingly exasperated with the government, which has lost three court cases over its perceived lack of action on air pollution.

It is a sentiment shared by Grigg who says it is “crazy” that ministers have still not taken decisive action to get the most polluting diesel vehicles off the road – even though they know the damage the vehicles are doing to the nation’s health.

But promises of action in the future may already be too late to persuade Correia – and thousands like her – to stay.

“We did not think about pollution before we became parents,” she says as she straps Madalena into a sling. “But now we are and we know about this, what else can we do?”