Poisonous air is having a devastating impact on billions of children around the world, damaging their intelligence and leading to hundreds of thousands of deaths, according to a report from the World Health Organization.

The study found that more than 90% of the world’s young people – 1.8 billion children – are breathing toxic air, storing up a public health time bomb for the next generation.

The WHO said medical experts in almost every field of children’s health are uncovering new evidence of the scale of the crisis in both rich and poor countries – from low birth weight to poor neurodevelopment, asthma to heart disease.

Dr Tedros Adhanom, WHO director general, said: “Polluted air is poisoning millions of children and ruining their lives. This is inexcusable – every child should be able to breathe clean air so they can grow and fulfil their potential.”

The findings coincide with the start of the first global conference on air pollution and health in Geneva, including a high-level action day at which nations and cities are expected to make new commitments to cut air pollution.

The WHO study found that children are particularly vulnerable to air pollution because pollutants are often more concentrated nearer to ground level. It added that their developing organs and nervous system are also more susceptible to long-term damage than those of adults.

“Air pollution is stunting our children’s brains, affecting their health in more ways than we suspected,” said Dr Maria Neira, WHO director of public health and the environment.

The study found that 600,000 children die from acute lower respiratory infections caused by dirty air and 93% are exposed to one of the most damaging pollutants – PM2.5. In poorer countries, 98% of all children under five are exposed to PM2.5 above WHO guidelines.

Pregnant women are particularly vulnerable, with dirty air linked to premature and underweight children. Air pollution also increases the risk of chronic diseases such as cardiovascular disease later in life.

Tedros, writing in the Guardian on Saturday, described air pollution as the “new tobacco”, saying the simple act of breathing is killing 7 million people a year and harming billions more.

In the UK, most urban areas have illegal levels of air pollution and ministers have lost three times in the high court after challenges over the inadequacy of their action. The latest government action plan, called “pitiful” by environmental lawyers, revealed air pollution was actually much worse than previously feared.

Globally, with smoking on the decline, air pollution now causes more deaths annually than tobacco. However, researchers think the harm known to be caused by air pollution, such as heart attacks and lung disease, is only “the tip of the iceberg”.

Today’s report found that both indoor and outdoor air pollution was causing significant health problems. It said burning fuel such as wood or paraffin for heating, cooking and light in poorer countries was having a drastic impact on children’s health and called on governments to promote the clean alternatives as a matter of urgency.

Neira said there were “many straightforward ways to reduce emissions of dangerous pollutants ”, including “accelerating the switch to clean cooking and heating fuels and technologies, [and] promoting the use of cleaner transport, energy-efficient housing and urban planning. We are preparing the ground for low-emission power generation, cleaner, safer industrial technologies and better municipal waste management.”

Mark Watts, executive director of the C40 Cities group, which represents cities around the world working to tackle the climate crisis and air pollution, said the report was an urgent call to action.

“The moral and practical case for urgent, bold and far-reaching action to reduce emissions, including calling an end to the fossil fuel era, is now utterly irrefutable,” he said.

“Citizens are demanding action to protect their children, mayors of the world’s big cities are regulating to take dirty vehicles off the streets and slash emissions from buildings and waste. Now is the moment for governments, car manufacturers and other big polluters to step up.”

Neira said the air pollution crisis and the climate emergency could only be tackled together.

“The solutions are a basic public health agenda that will have plenty of benefits for public health and the environment,” she said. “No matter what else, we know that we need to decarbonise our society sooner rather than later and the benefits of that for our health and our economy are indisputable.”