The top 100 most polluted cities in the world are all in Asia. Severe air pollution persisted in major Indian cities, including New Delhi, and parts of Pakistan in November 2024. Local residents were advised to stay indoors, and schools and outdoor construction work were suspended as a result of winter smog events.

But air pollution is nothing new 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 relies on wood fires for heating.

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

What causes air pollution and how does it become smog?

Smog is a portmanteau of “smoke” and “fog”. That should give you an indication of how this dirty, chemical fog appears.

It forms when ground-level pollutants such as ozone, particulate matter, sulfates, nitrates, and other toxic chemicals combine with haze 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 plant, your car engine, or your wood-burning stove—release toxic gases into the atmosphere.

Often suspended in smoke and gas are microscopic particles that result from 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 micrometers
• PM2.5 for particles 2.5 micrometers or less
• PM0.1 for ultrafine particles of less than 100 nanometers

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

Bacteria, such as disease-causing bacteria E.coli, they are about 3 micrometers wide, so PM2.5 is even smaller than that.

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

Because of their microscopic size, inhaling large numbers of these chemical particles—consisting of toxic gases, heavy metals, and volatile organic compounds—can easily be absorbed into the bloodstream, where they can cause long-term damage.

What are the health effects of smog and air pollution?

Inhalation of polluting particles and 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 infections, and can affect lung function.

In the long term, chronic conditions can occur, including cancer, strokes, heart disease, and obstructive lung disease.

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

In May 2024, a study of low emission areas in Germany found that children who were exposed to clean air from conception to the first year were less likely to need medication before the age of five.

“Being exposed to air pollution at this very early stage of life can have longer-term effects as children grow,” said Hannah Klauber, the study’s lead researcher.

Previous studies have also shown that children who are exposed to pollution early in life do less well in school, get lower test scores and, on average, earn less income as adults.

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

While Klauber’s study focused only on Germany, Klauber said he would expect to find similar results elsewhere in the world.

How is air quality rated and why?

Air quality assessments 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 a lot is based on global recommendations from the World Health Organization.

Some countries and cities codify their quality ratings. For example, in the US and India:

• Green is for good quality air
• Yellow is for moderate pollution
• Orange for is bad 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 against smog if you find yourself in a situation where you cannot avoid it.

But in some cities with high pollution, such as New Delhi and Lahore, authorities are imposing restrictions on outdoor activities. This includes school closures, limits on driving cars and other vehicles, and suspension of outdoor work.

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

Do closing schools help reduce air pollution?

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

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

Actions are being taken around the globe to impose stricter limits on air pollution. The European Union agreed new standards in June 2024, and in Asia efforts to reduce air pollution are also underway in some of the worst-hit places, such as Beijing, China.

Beijing authorities introduced a plan to electrify public transport services in 2013. This has led to some significant reductions in smog and pollution, but levels are still above government and global air quality recommendations.

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

Edited by: Zulfikar Abbany

Select sources:

WHO Global Air Quality Guidelines (2021)

Study: Killing Prescriptions Softly 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