Why Some Cities Choke on Smog Every Winter

On some winter mornings, the buildings across the road disappear into a grey-brown haze. Flights are delayed, schools close and hospitals see more people with breathing problems. It happens in Delhi and Lahore, in Beijing, in Kraków in Poland, and even in Salt Lake City in the United States.

These cities don’t suddenly have ten times more cars in winter. So what changes? The short answer is the weather. Many cities produce a lot of pollution all year round. In winter, under the right conditions, it stops going anywhere.

The lid in the sky: temperature inversions

Normally, air is warmest near the ground and gets cooler as you go up. Warm air rises, carrying smoke and exhaust upwards and spreading them through a deep layer of the atmosphere.

On clear, calm winter nights, the ground loses heat quickly. The air touching it cools, while the air higher up stays relatively warm. This flips the usual pattern and is called a temperature inversion.

Cool air is heavier than warm air, so it stays put near the ground. The warmer layer above acts like a lid. Smoke, dust and exhaust are trapped in a shallow layer, sometimes only a few hundred metres deep, right where people live and breathe. In winter, the weak sun may not warm the ground enough to break the inversion during the day, so it can last for days.

Many cities produce a lot of pollution all year round. In winter, it stops going anywhere.

Why valleys and basins suffer most

Geography makes a big difference.

• Valleys and basins trap cold air like water in a bowl. Salt Lake City sits in a basin ringed by mountains, and in winter its air can be among the worst in the US for days at a time. Kraków lies in a river valley, and Beijing has mountains to its north and west.
• Wide plains next to mountains behave in a similar way on a much bigger scale. The Indo-Gangetic plain of northern India and Pakistan, home to hundreds of millions of people, is bordered by the Himalayas. With light winter winds and a mountain wall on one side, polluted air builds up across the whole region.
• Still air makes everything worse. Without wind, polluted air isn’t blown away or diluted. Rain and strong winds usually bring a few days of cleaner air.

Where the pollution comes from

Winter smog rarely has one cause. The main sources usually include:

• Heating, especially burning coal and wood in homes. In Kraków and many cities in Central and Eastern Europe, household coal and wood stoves have been one of the biggest sources of winter smog.
• Vehicles, particularly older diesel engines and heavy traffic
• Industry and power plants in and around the city
• Road and construction dust
• Burning of waste and farm crop residue. In northern India, smoke from crop fires in late October and November adds to the problem on some days.
• Fireworks at festivals and celebrations, which cause short, sharp spikes

The share of each source changes from city to city and from day to day.

Gases that turn into particles

Not all fine particles are released directly. Gases such as sulphur dioxide, nitrogen oxides and ammonia react in the air to form new tiny particles, called secondary particles. Cold, humid conditions help these reactions and help particles absorb water and grow, which makes the haze thicker. When fog mixes with smoke, you get smog: the word itself is a blend of the two.

Two kinds of smog

Winter smog is different from the brownish summer smog of cities such as Los Angeles or Mexico City. Summer smog forms when sunlight reacts with vehicle exhaust to make ozone and other pollutants, and it is worst on hot, sunny afternoons. Winter smog is mostly smoke and fine particles trapped by cold, still air, and it is usually worst at night and early in the morning.

Cities that cleared their air

Winter smog isn’t inevitable. In December 1952, a thick smog from coal fires covered London for five days. It caused around 4,000 deaths at the time, and later studies suggest the true toll was far higher. The UK passed the Clean Air Act in 1956, which created smoke-free zones and moved homes away from burning coal.

More recently, Beijing cut its average PM2.5 levels sharply after 2013 by replacing coal heating and coal power, controlling traffic and moving heavy industry. Poland’s Kraków banned burning coal and wood in home stoves from 2019. Many cities now use alert systems, such as the Graded Response Action Plan in Delhi, that bring in stricter limits on traffic, construction and burning as the air worsens.

Protecting yourself on bad-air days

• Check your local air quality index and plan outdoor exercise for when it’s lower, often the afternoon in winter.
• Keep windows closed during the worst hours, usually late at night and early in the morning.
• An air purifier with a HEPA filter can reduce fine particles in a closed room.
• If you have asthma, a heart or lung condition, or are pregnant, ask your doctor for advice on masks and medicines.