Before heading off on a foreign assignment, journalists take a course about working in hostile environments — learning about things like trauma first aid, weapons effects, and how to survive earthquakes, floods and civil unrest.

It's all pretty useful training. And heading off to live and work in India, I was more than aware of the everyday dangers I'd be facing.

For instance, India has one of the world's highest road tolls and Delhi is one of the worst places in the world for sexual violence against women.

But I had no idea the most hostile thing I would encounter upon moving to India would be the air I'd have to breathe every day.

Vehicles drive through smog in New Delhi. ( Reuters: Saumya Khandelwal )

The reality of breathing in Delhi

Within a week of moving to Delhi, the first signs of trouble started to show.

I would wake up in the middle of the night with my throat on fire. I'd lay awake for hours unable to soothe the pain.

My partner suffered worse — he developed a chest infection and a disgusting cough.

I'd read plenty of news stories about air pollution in Delhi; that it's been getting worse for the past three years, that it peaks in winter, that sometimes schools are shut and that breathing the air is the equivalent of smoking 50 cigarettes a day.

Late last year, when the smog was at its worst, Delhi's Chief Minister called the city a "gas chamber", and doctors declared a public health emergency.

OK, I thought, I'll wear a mask — I'll get a good one.

Air that takes years off your life

Until you're breathing the air, until you're tasting those chemicals on your tongue, until your eyes start to water uncontrollably when you go outside, it's difficult to really understand how it's going to affect your life.

So let me explain exactly what people in Delhi are sucking into their lungs.

The big problem is a miniscule airborne particle — so tiny you could fit 30 across the width of a human hair.

Being so small, they can travel deep into a human lung, and from there into the bloodstream.

There the toxins can cause respiratory illness, heart attacks and cancer.

Sri Lanka's players wear anti-pollution masks during a game of cricket in Delhi. ( AP: Altaf Qadri )

Scientists say people who live their whole lives in Delhi can expect to shave a decade off their lives thanks to air pollution.

I used to check Twitter on my phone first thing every morning.

Now the first thing I check is one of the many air quality apps which send real-time data to your phone about how polluted the air is.

There are several categories for air quality, ranging from good and moderate to unhealthy. Then there's very unhealthy.

Then there's hazardous — "outdoor not recommended", it warns. Then there's "beyond the air quality index". That's when the air is so polluted it can't be measured.

Many residents of Delhi have no choice but to slowly choke on the toxic air their city serves up. ( ABC News: Siobhan Heanue )

According to guidelines set by the World Health Organisation, when the concentration of these tiny particles in the air is greater than about "25 micrograms per cubic metre", there are unhealthy side effects.

In Delhi, it's commonly around 200 micrograms per cubic metre, and I've seen it reach as high as 520 in the short while I've been here.

What's the solution?

The solution, for those who can afford it, is buying high-tech air purifiers for the rooms you spend the most time in. Slick, wi-fi-enabled Swedish air purifiers.

They whirr all day and night — a gentle but persistent reminder of the putrid air they're processing.

Every morning many people wake up and check their phone to see how polluted the air is. ( AP: Altaf Qadri )

But breathing purified air creates its own problems. Because the air is constantly recycled, the level of oxygen is slowly reduced and the level of carbon dioxide increases.

Eventually the CO2 makes you sleepy. Your brain gets fuzzy. Then you have a choice.

Open a window to let in some fresh oxygen along with a fresh dose of poisonous air, or deal with the clean but doze-inducing air already inside.

But at least I have a choice.

For millions of people in this mega-city, there's no such thing.

The street-hawkers, the chai-wallahs, the tiny kids who beg at the intersections, the street guards, the traffic police, the children walking to and from school every day — they have no choice but to slowly choke on the toxic air their city serves up.

A study reported that a quarter of all premature deaths in India in 2015 — or 2.5 million — could be attributed to pollution. ( Wikimedia Commons )

From environmental crisis to humanitarian catastrophe

A study in the Lancet medical journal reported that a quarter of all premature deaths in India in 2015 — or 2.5 million — could be attributed to pollution.

But there seems to be no political will or plan to address the pollution problem.

Last month there was a theatrical display of water guns rolled out into a few city streets to "shoot down" the dust.

This week the Government announced it was setting up a "war room" in its pollution control department to monitor developments. But it's all symbolic.

These last few years of worsening air portend a far more dangerous, even dystopian, future.

Without intervention, this environmental crisis will become a humanitarian catastrophe.

No matter how shocking and restricting I find the pollution, at the back of my mind I know that eventually I get to leave.

For most of Delhi's 20 million residents, that's not an option.