Beijing has again been blanketed in smog, with authorities issuing the second-highest level alert and advising residents to stay indoors.
It comes as international leaders, including Chinese President Xi Jinping, are meeting in Paris for talks on climate change.
Andrea Nierhoff reports.
It's another grey and hazy day in the Chinese capital Beijing, with heavy smog covering the city.
In recent weeks, concentrations of tiny airborne particles which can embed themselves in the lungs have reached extreme levels, well above the World Health Organisation's safe guidelines.
This resident says the effects of the pollution are instantly noticeable.
(Translated)"This sort of weather, you can see that all of Beijing has been enveloped in smog, and for every breath, getting up every morning, your throat will feel particularly uncomfortable, somewhat prickly, and your breathing is greatly affected."
China's poisonous winter smog is well known, with the use of coal burning in factories and homes, as well as heating, causing it to spike.
Authorities in Beijing issued an orange alert, the second-highest pollution level, advising people to stay inside as much as possible.
Dr Angela Pratt, who works for the WHO, lives in Beijing, and says the reasons for the fumes vary.
"It's hard to pinpoint a particular reason for a particularly high level of pollution on any given day. Really the causes are broadly the same causes that we see elsewhere in the world: high-polluting factories, coal-fired power stations, lots of cars on the road."
She says more and more people are demanding something be done about the issue.
"On the other hand there is an increasing level of awareness here in China about how harmful high levels of pollution are: so for example people do tend to stay indoors unless they really need to be outside, people wear face masks to protect themselves from the pollution if they are outside."
Beijing resident Shang Jing agrees, saying she thinks government-enforced guidelines might be the best step.
(Translated) "I hope, well, personally I feel that the first step might be that we must find what creates this terrible pollution problem, and if it has anything to do with cars or industrial manufacturing, then maybe we could impose limitations on vehicles, or perhaps somehow deal with our industrial manufacturing."
Dr Pratt says the health effects of living with the smog are numerous, including respiratory problems and even cancer.
But she believes the government is taking it seriously, and changes are happening.
"There are short term and long term health effects of exposure to air pollution. Short term: respiratory problems, breathing problems, people who are prone for example to asthma attacks, and I think if you looked at the data from hospital admissions, for example, you would see some short term spikes in some of those areas. Long term air pollution does cause health problems, again chronic respiratory problems, heart problems, we know that air pollution is carcinogenic, we know that prolonged exposure can cause lung cancer. Things are changing, the Chinese government is acting on these issues and we are seeing some improvements. It's not an area where there will be quick improvements because it's a very big and complex area."
She says people don't notice when Beijing experiences good weather days, but focus just on the negative.
"There are in fact a lot of days where it is like that. Obviously it's not good today and I understand there's a news interest on the days when the pollution does get very heavy, but there are lots of days where it's not like that, where we see the blue skies as you saw during the Olympics."
China is estimated to have released between nine and 10 billion tonnes of carbon dioxide in 2013 alone.
It has pledged to cap emissions by around 2030, suggesting at least another decade of pollution.
The Ministry of Environmental Protection has forecast severe pollution for Beijing and parts of northern China until Tuesday, when strong winds are expected to help clear some of the smog.
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