The 100 most polluted cities in the world are all in Asia. Severe air pollution persists across South Asia. In November 2024, residents of major Indian cities, including the capital New Delhi, and parts of Pakistan were asked to stay indoors, and schools and outdoor construction work were suspended due to smog incidents.

Such air pollution is common in the world’s most populated and built-up cities. And exposure to polluted air can happen anywhere: Whether a person is walking through a city full of factories, stuck in rush hour traffic, or in a rural area that depends on wood fires for heating.

However, for many people in the worst affected parts of the world, taking precautions to avoid poor quality air is easier said than done.

What causes air pollution and how does it become smog?

Smog is a portmanteau of “smoke” and “fog,” giving a first indication of how this dirty, chemical haze occurs.

It forms when ground-level pollutants, such as ozone, particulates, sulfates, nitrates, and other toxic chemicals combine with fog under sunlight.

Why are smog and air pollution dangerous?

Smog and pollution are dangerous because they are so easily inhaled.

Combustion processes – whether in an industrial factory, the engine of your car or the stove for your wood burning home – release toxic gases into the atmosphere.

Often suspended in the smoke and gas are microscopic particles that are the result of complex chemical reactions between the substances we burn.

Particles are labeled by size. For example:

• PM10 for particles with a size of 2.5-10 micrometres
• PM2.5 for particles 2.5 micrometers or smaller
• PM0.1 for ultrafine particles of less than 100 nanometers

These particles are small. By comparison, a human red blood cell would fit within the PM10 size range, as they are about 6-8 micrometers in diameter.

Bacteria, such as the disease-causing type E. coli, are about 3 micrometers wide, making PM2.5 even smaller.

As for ultrafine PM0.1, these particles are smaller than the viruses that cause influenza and HIV.

It is because of their microscopic size that breathing in a lungful of these chemical particles – made up of toxic gases, heavy metals and volatile organic compounds – means they can be easily absorbed into the bloodstream, where they can cause long-term damage.

What are the effects of smog and air pollution on health?

Inhalation of particulates and pollutant gases has long been associated with poor health and a range of diseases and disorders.

Short-term exposure can worsen acute conditions, such as asthma and other respiratory problems, and can impair lung function.

In the longer term, chronic conditions can occur, including cancer, stroke, heart and obstructive lung diseases.

This can affect people of all ages, but children and individuals over 65 are particularly vulnerable.

In May 2024, a study of low emission zones in Germany found children exposed to clean air from conception to their first year needed less medication before age five.

“Exposure to air pollution during this very early period of life can have long-term effects as children get older,” said Hannah Klauber, the study’s lead researcher.

Previous studies have also shown that children exposed to pollution early in life perform worse in school, achieve lower test scores and, on average, earn lower incomes as adults.

“We have seen in several studies that there are no safe levels of air pollution,” Klauber told DW. “There are essentially no safe levels of particulate matter, so any increase in particulate matter leads to adverse health effects.”

While Klauber’s study only focused on Germany, Klauber said she expects to find similar results elsewhere in the world as well.

How is air quality assessed and why?

Air quality ratings are used to monitor the standard of air pollution in an area.

Such rating scales are usually developed by national governments, so standards may vary from country to country. But many build on global recommendations from the World Health Organization.

Some countries and cities color code their quality ratings. For example in the US and India:

• Green is for good air quality
• Yellow is for moderate contamination
• Orange is for poor air quality
• Red is for very poor air quality

What can you do to protect yourself from smog?

There is little you can do to effectively protect yourself from smog other than avoid it as much as possible.

In some cities with high pollution, such as New Delhi and Lahore, authorities impose restrictions on outdoor activities. It includes closing schools, restrictions on driving cars and other vehicles, and suspension of outdoor work.

Cities prone to smog and high levels of air pollution may also advise residents to use filtration mechanisms where possible, and reduce physical exertion.

Do school closures help reduce air pollution?

No, not according to Rajib Dasgupta, professor of public health at New Delhi’s Jawaharlal Nehru University. Dasgupta told DW that restrictions on outdoor activities or school closures were just stopgap.

“It’s something you can’t really deal with through personal or household interventions. It’s something that has to involve government action and very large multi-sectoral action,” Dasgupta said.

Actions are being taken around the world to impose stricter limits on air pollution. The European Union agreed to new standards in June 2024, and in Asia efforts are also underway to reduce air pollution in some of the most affected areas.

Authorities in Beijing introduced a plan to electrify public transport in 2013. It has seen some significant reductions in smog and pollution, but levels remain above government and global air quality recommendations.

India has also adopted new clean air policies, but Dasgupta criticized the lack of progress: “States can’t seem to get their act together, and it’s not for lack of money, but for lack of will.”

Edited by: Zulfikar Abbany

Select sources:

WHO global air quality guidelines (2021)

Study: Killing Prescriptions Softly, published by Klauber, Hannah, Felix Holub, Nicolas Koch, Nico Pestel, Nolan Ritter and Alexander Rohlf in American Economic Journal, Economic Policy (2024) DOI: 10.1257/pol.20210729