I smoked 30 cigarettes a day for a year

The first time it’s a mystery. You wake up with a headache, your eyes itch, and you feel nauseous. Then you call a friend to complain, and they laugh. It’s just living in Delhi, the city with the worst air on the planet.

A simple way I used to explain what it was like to people who hadn’t experienced the air in Delhi was to say imagine if you felt carsick the whole time, with a metallic taste on your tongue after any decent amount of time spent outside.

As the weather gets colder, the smog gathers. You sometimes can’t see the car in front of you thanks to it, while sports matches get cancelled for players not being able to look down the length of the pitch. The Chief Minister of Delhi even described his city as a “gas chamber”.

Air quality is usually measured by how much particulate matter can be detected in the air, specifically matter smaller than a human hair. The worst is called PM2.5, particles that are even smaller than pollen and are usually made up of the consequences of burning things: fields, metals, chemicals.

PM2.5 is so small that it’s tough for your body to stop the particles coming in, and so you end up with a nice pile of ash from a Delhi chemical plant in your lungs. That’s why you’re coughing.

To help people understand the risks of bad air, the US Government developed the Air Quality Index. Sensors measure how much particulate matter is in the air and convert it into a number out of 500, divided into ranges of risk.

Central London right now for instance is 35. New York is 60. Beijing 151. The AQI measured at the US Embassy in Delhi this morning was 704.

Sometimes it will even go above 1,000, double the US Government’s worst-case scenario for air quality. But most people will never know, as filters and monitors tend only to be able to reach 999.

The horrendous air is a relatively recent phenomenon as the population and economy of India have boomed. Still, it is having a health impact on the 20 million who live in Delhi and its surroundings. Small studies are already beginning to show an increased prevalence of cancer and other conditions, and this will only get worse. Supposedly, spending a day breathing in the fresh Delhi air is equivalent to smoking around 30 cigarettes a day.

But I was lucky; I could afford an air purifier. I also only lived there for a short period of time. For everyone who can’t, it’s slowly killing them. Even with the air filters, the metallic smell lingers, seeping its way through the gaps around your doors. People do the best they can with what they can afford, often even buying plants for their homes to add more oxygen, but you can’t escape having to breathe.

When the weather heats up again, and the farmers stop burning fields, the air quality improves. Everyone then tends to forget about it, until their Delhi Cough comes back.