Sky Views: When the air is not safe to breathe

Katie Stallard, Asia Correspondent

Since the start of the year we've been living under a cloud.
It's a toxic cloud, comprised of air pollutants from heavy industry, coal-fired central heating, and car exhausts, and featuring a particularly nasty little particle called PM2.5, a known carcinogen.
For much of the last month the air quality in Beijing, and more than 20 other cities in China's north-east, has ranged from "unhealthy" to "very unhealthy" to "hazardous".
In places it has registered off the scale.
When it's like this, it's not safe for children to play outside.
It's not much fun for adults either.
Articles repeatedly liken Beijing's smog to "an airport smoking lounge" - and honestly, it doesn't feel too far off.
You can smell the pollution, at times even taste it. I have a recurring itching sensation in my throat.
On the worst days, schools and factories have been closed, hundreds of flights cancelled, up to half the capital's cars ordered off the roads.
You find yourself leading an almost hermetically-sealed existence: wearing a mask to move from inside one building to the next, going to sleep to the whirr of the air purifier, a fan-like device designed to suck the worst of the particulates out of the room.
This is not a complaint.
All of the above is a measure of how relatively fortunate I am - that I have a high-tech, whirring air purifier to sleep next to, a decent mask to wear outside, and a colleague who has painstakingly sealed the gaps around our office windows.

There are plenty of people here with none of these, and in the absence of a wide-scale public education campaign, less than reliable information about how to protect themselves and their families from the toxic smog.
It's not uncommon to see children on their way to school in paper surgical or dust masks - which give them no protection from the smallest, most dangerous particles.
Air pollution is not a uniquely Chinese problem.
The UK had its own hazardous smog during the industrial revolution of the 19th century.
Beijing could argue this is just its 21st century version - the cost of developing, and lifting millions of its citizens out of poverty.
But this is not the 19th century - we know more now about the effects, and how many of those citizens this smog is killing.
A World Health Organisation report, published last September, ranked China as the world's deadliest country, by far, for air pollution - with more than one million related deaths in a single year (2012).
This compared to 621,000 in India and 140,000 in Russia for the same year.
The figure for the UK was 16,355.
Another study, published in 2015 by physicists at the University of California, estimated China's air pollution was killing around 4,000 people a day.
You would expect death on this scale to trigger a concerted nationwide response, and China's leaders did duly declare a "war on pollution" in 2014, but progress in that battle appears to be wavering.
After two years of fairly steady improvement in air quality in the worst affected Beijing-Tianjin-Hebei region, in March 2016 that trend started to reverse.
Greenpeace charted the deterioration of air quality over the course of the next nine months. 
Despite claims by the government that there were more good days in 2016, by November it was significantly worse than the same month last year.
Their research found a striking correlation between air pollution and the output of iron and steel, which they attribute to a massive government stimulus package earlier in the year.

People sometimes joke on polluted days here that you can smell the GDP in the air, and perhaps that's not too far from the truth.
Faced with the option of fuelling continued economic growth, and consequent support for the Communist Party - it is a brave official who will choose to put China's environmental issues first.