Hong Kong has an air pollution problem. If the territory conformed to the World Health Organisation's (WHO) standards for clean air, only 10 per cent of the days here would pass muster.

According to the WHO, air pollution is "the world's single largest environmental risk". In October last year, the WHO's International Agency for Research on Cancer (IARC) classified air pollution as a group 1 carcinogen for the first time. This puts it in the same category as smoking cigarettes, with "sufficient evidence" that exposure causes lung cancer. The IARC also classified particulate matter (PM), a major component of the air pollution in Hong Kong, as carcinogenic.

Lung cancer is only one of many negative outcomes. Air pollution both causes and exacerbates ongoing irritations such as coughs, mucous build-up and inflamed airways. It also leads to more life-threatening situations such as cardiovascular disease and heart attacks. Vulnerable groups, including children, the elderly and people with existing heart and respiratory conditions, are most at risk. Some people are genetically predisposed to be more sensitive to pollutants. Those who spend a lot of time outdoors are also more likely to succumb to health problems. The more you delve into the subject, the more our pollution-tinged skies appear ominous.

"Air pollution is certainly the most important public health risk in Hong Kong," says Dr Tian Linwei, associate professor at the School of Public Health, at the University of Hong Kong. "If you look at environmental issues of air, water and solid waste, the quality of the air stands out as being well below standard. And the air quality is worsening. Even without any pollution data, we can simply see that the visibility is getting worse. The government says it is improving, and it is in some respects, for example, the PM 10 levels. However, overall it has deteriorated."

In public-health terms, air pollution is defined as having both a chronic, long-term effect and an acute, short-term effect. The acute effect day-by-day, month-by-month, eventually merges into the chronic effect.

"There are a lot of studies in Hong Kong that demonstrate the link between air pollution and hospital admissions, in other words an acute effect," says Tian. "Data about the long-term effect of pollution on health is not readily available as a population-based study requires a lot of resources and infrastructure. Globally, there have only been about five meaningful studies, in America and Europe. They have three consistent findings. First that air pollution shortens life expectancy. Second, that a high level of the smallest particles in air pollution is related to lung cancer risk. Third, these small particles are also related to cardiovascular disease and mortality. The higher the levels of particles in the pollution, the higher the risk."

The Environmental Protection Department (EPD) monitors pollution in Hong Kong at 12 general and three roadside stations. It takes readings on the four main pollutants and then calculates the short-term health risk. This information is made available as the Air Quality Health Index (AQHI) online and via an app. It's graded into a scale from one to 10+. One indicates low risk to health and 10 a serious risk. The current health advice is to reduce activity and time outdoors when the pollution hits seven on the scale. By 10, you should be indoors.

The four main pollutants measured are sulphur dioxide (SO2), nitrogen dioxide (NO2), ozone and airborne particle matter.

SO2 and NO2 are gases caused by combustion in power plants, industrial boilers and vehicle engines on land and at sea. They lower the body's resistance to respiratory disease, aggravate existing disease and irritate and damage the respiratory system. They can cause cardiovascular disease in people with sensitive respiratory systems, including asthmatics, the elderly and the young. They also impair lung development in young children.

Ozone is formed when NO2 reacts with hydrocarbons in the presence of sunlight. Ozone irritates the mucous membranes in the nose, throat and airways, which causes eye irritation, coughs, and throat and chest pain, inflames the respiratory system and decreases lung function.

Airborne particle matter is made of tiny particles of solid or liquid substances suspended in the air. They are defined by size rather than substance. PM 10 particles have a diameter of less than 10 micrometers and PM 2.5 have a diameter of less than 2.5 micrometers.

The human nose does a relatively good job of filtering particles larger than PM 10. Smaller than that and it fails, allowing the particles direct access to our lungs. The very smallest, or ultrafine, particles have been found to penetrate not just the lungs but also cell membranes, which means they can get into other organs, including the brain. These particles are dangerous not just because of their size but also their chemical composition, which, in some cases, is carcinogenic.

The effect of inhaling particulate matter has been widely studied and documented, with depressing results: blocked airways, coughs, decreased lung function, aggravated asthma, lung cancer, cardiovascular disease, respiratory disease, birth defects, premature delivery and premature death.

Currently, the number of days deemed dangerous to health using Hong Kong's AQHI stands at about 30 per cent. (Hong Kong's ambient air guidelines are less stringent than those of the WHO.)

Last year, the Hong Kong government came up with a Clean Air Plan, to reduce locally produced vehicle, marine and power plant emissions. It is widely considered to be a good plan. The government is also working at a regional level with the Guangdong authorities to reduce emissions in the Pearl River Delta.

While we wait patiently for these policies to bear fruit, is there anything else that we can do to mitigate the pollution?

BEIJING, HONG KONG'S ultimate seat of power, chokes under a far thicker fug than that of the SAR. In the Chinese capital, it is common to see people wearing facemasks to filter out the smog.

Christopher Dobbing is director of Vogmask China. Vogmask is an anti-pollution facemask designed for babies, children and adults. The firm was started in California, in the United States, two years ago, producing masks developed to protect partygoers from dust at the Burning Man festival, in the Nevada Desert. The South Korean-made products are classified as removing at least 99 per cent of airborne particles and have been on sale in Beijing for 18 months, retailing at between HK$230 and HK$285.

"The average northern Chinese resident lives about five years less than a Chinese person living in the south and that's down to air pollution," says Dobbing. "Here in Beijing, doctors recommend wearing a mask at around 200 AQI, which is equivalent to level 7 in Hong Kong."

The whole mask is a filter. "If it's above 300 AQI, the mask would last 80 hours. If you're in a much less polluted environment, say, like Hong Kong, you'd expect it to last for 300 or 400 hours of use." Once that time is up, you dispose of the mask and replace it.

"The needs in Hong Kong are different from those in Beijing. If you are cycling in traffic or have a respiratory illness or sensitivity to pollen or dust, then a mask is a good non-medical solution. It will take a generation to clear the pollution up. Over that time, wearing a face mask and running an air purifier indoors is something everyone should be doing."

"There have been a lot of studies on face masks," counters Tian. "They prevent large particles. However, our noses also stop large particles. If you want to stop small particles, you need a very, very good mask, which takes a lot of pressure to breathe through. This can cause another kind of damage, which can be dangerous for the elderly and children."

AIR PURIFICATION IS big business in Hong Kong. Hospitals, businesses, schools and homeowners buy air purifiers in the hope they will render indoor air safe. IQAir manufactures air purifiers in Switzerland. It is a well-known brand, not least because of its hefty price tag. Its best-selling model, the HealthPro 250, sells for about HK$15,000.

"In the basic model, there are two filters," says Amy Yeung Mei, marketing manager for Airtek, the sole agent of IQAir in Hong Kong. "The first is the pre-filter, which tackles the large airborne particles like PM 10 and dust particles. The second filter removes smaller particles like pollen, fungi, mould spores, bacteria and allergens. It takes out up to 99.97 per cent of 0.3 micrometer."

Some manufacturers, including IQAir, claim their devices remove organic vapours, for example formaldehyde, but this is difficult to confirm scientifically.

The machines can run on a similar amount of energy as a light bulb. One machine is needed in each room of an apartment or house.

Dane Westerdahl is a research scientist at the School of Mechanical and Aerospace Engineering at Cornell University, in the US, with a consulting role at the School of Energy and Environment at City University of Hong Kong. He has undertaken many air pollution studies, in cities including Jakarta, Beijing, Milan, Barcelona and Los Angeles.

"I feel sorry for people who are trying to do the best thing by buying an air purifier, because the marketing is not good," he says. "It is very hard to be a well-informed consumer, no matter how sophisticated you are. You are making an assumption that buying the most expensive thing will be the most effective solution. It may be the best because it may be more durable but the fact is that inside these machines, [the technology is] very simple. They have filters to take out different sized particles. Coarse particles like dust, medium particles like pollen and small particles like smoke. Ninety per cent of these air purifiers are the same inside. They've got a fan and two or three levels of filter material. Personally, I would advise people to look for a [brand] they recognise from their household appliances in a price range they can afford. Most importantly, remember air filtration systems only work if you have the right sized machine for the room and change the filter regularly. Otherwise they are pretty useless."

Westerdahl uses two second-hand air purifiers, which he bought for HK$500 each, in his apartment in Sham Shui Po.

"I have worked on pollution for 40 years and I am concerned enough about the air here to have two of them. You can make a difference with an air purifier in your home. I ran tests on the air purifier in my bedroom. Within 10 minutes of turning it on, the particle level reduced by about 95 per cent."

His advice is to research what sized particles the machine can remove: "The particles from diesel are 0.2 to 0.3 micrometers. In this city, that's the size particles you want to get rid of. They also happen to be the hardest to remove because of the physics of filtration. You want to check that whatever you buy specifies that it removes these very small particles."

For those who cannot afford an air-purifying machine, Westerdahl suggests putting filtration material, for example, 3M Filtrete, which you can buy in hardware stores, into the filters of your air conditioner.

"I ran a test in my own home by putting filtration material into my window air conditioner. After two weeks' use, the colour changed from white to black. The kind of pollution this is catching has a lot of carbon in it. It's diesel exhaust, a traffic-related pollutant. If the filter isn't catching this then your lungs will. You don't have to spend a fortune to make a difference."

IT IS ALSO POSSIBLE to clean indoor air using plants. In 1989, Nasa and the Associated Landscape Contractors of America investigated how to clean the air in space stations. The resultant Clean Air Study contained a list of air-filtering plants. Plants absorb carbon dioxide and release oxygen. Some also filter out volatile organic compounds (VOCs), for example, formaldehyde, which is carcinogenic and a component of indoor air pollution.

Kamal Meattle is an entrepreneur, environmentalist and chief executive of Paharpur Business Centre, in the Indian capital, New Delhi, the most polluted city on the planet. In 2009, he gave a TED talk titled, "How to Grow Your Own Fresh Air", which is available on YouTube.

Twenty years ago, in response to his polluted environment, Meattle set about creating a clean working space.

"Indoor air pollution is a slow and invisible killer," he says. "On average, we take 23,000 breaths a day, about 12,000 litres of air. Our brain utilises approximately 20 per cent of the body's oxygen supply and brain cells quickly begin to die when they are deprived of the oxygen, leading to loss of memory, sleep apnea, fatigue and more. Moreover, we spend more than 90 per cent of our time indoors. If we fix our office and our home, it will give our respiratory system the rest it needs to build immunity to fight pollution outdoors. Hence, it is imperative that we look at the solutions that keep us healthy in our indoor spaces."

Meattle consulted the Nasa list and created natural air purification that conforms to WHO guidelines.

"We grow fresh air in our business centre with the help of more than 1,200 plants," says Meattle. "There are three easy-to-maintain plants that we use for natural air purification. They are the areca palm, mother-in-law's tongue and money plant. The areca palm and money plant produce oxygen during the daytime while mother-in-law's tongue produces oxygen during nighttime. These three common houseplants remove chemical toxins from the indoor air, reduce CO2 levels and enrich it with more oxygen. By keeping these plants, we ensure that there is a constant supply of oxygen in the air."

However, the volume of plants needed - between six and eight per person, per room - could make the average homeowner baulk. Meattle concedes that plants alone won't clean the air completely.

"Plants and air purifiers have a different mechanism. An air purifier is more effective at removing particulate matter and gaseous pollutants and the plants are more effective at reducing CO2 and in oxygenating the indoor air," he says. "However, one advantage of plants over air purifiers is that they detoxify air without consuming electricity. To have an effective solution to air pollution, one must use both air purifying plants and the air purifiers strategically."

Peter Brimblecombe is chair professor of atmospheric environment at City University of Hong Kong. "The problem is ... plants also give off things like pollen, terpenes and isoprene," he says.

"All these things are quite reactive. So, you can also produce air pollution with plants, which you have to be careful about. In outdoor plantings now, much more attention is paid to what plants produce. Atlanta, Georgia [in the US], for example, has a lot of air pollution generated by its surrounding forests."

Brimblecombe thinks the whole idea of air pollution monitoring needs turning on its head.

"It's difficult to lead a modern life and not be exposed to air pollutants," he says. "People should realise that living in Hong Kong exposes them to more air pollution than many other places. However, there's very little account being taken of where you are exposed to pollution. The monitoring stations are all at street level or slightly above, whereas many people live at 30 floors up."

While we cannot control the factories producing pollution in Guangdong province, we can modify our personal exposure: "There are simple lifestyle ways to do this. Choosing the way you walk and the time of day you walk somewhere, using the MTR instead of a bus, staying indoors and not ventilating your rooms when the pollution is high, reducing indoor sources of pollution, for example, choosing furniture and paint carefully.

"There are attitudinal responses and choices. Not everyone looks at the AQHI to see what the measurements are each day. That is the first step. Try to be specific and identify which pollutant is causing ill effect and then monitor that particular pollutant. If you notice a difference, adapt your exposure.

"Personal choice is a very important thing."

LOUISE BUCKLEY, a naturopathic and nutritional therapist at The Body Group, takes a different approach to beating pollution.

"I don't look at apps," she says. "If you walk around Hong Kong thinking, 'I can't breathe this horrible air', your body will react to that. Whereas, if you walk round Hong Kong thinking, 'I am safe and I am enjoying what I am living in', your body will react to that and your immune system will react differently. Most people who have height-ened immune systems also have heightened emotional reactions to things. The wrong attitude is worse than brea-thing air from China. A huge part of it is how you react.

"The first step is to change your attitude. If it's smoggy, I support my digestive and immune system. Take Rescue Remedy to help calm you down, drink calming teas. We should be more centred on what we could be doing as individuals, our own nutrient levels and our own physical and emotional health than worrying about an external thing that we have so little control over," she says.

"I am not a fan of air purifiers at all because they create a sterile environment that we're not actually built to survive in," says Buckley. "You take away bacteria and natural particles in the air, you change the very structure of the air around you. Your body starts to get very stressed about that because we've got our own mechanisms to do that filtration. Instead of thinking, 'Let's get a machine to do this for us', let's support our lungs. Let's use nutrients that support the mucus membranes in our lungs so that we're not so sensitive to the dry dust.

"Bacteria are hugely necessary in our bodies," says Buckley. "Bacteria are the original immune system. Instead of trying to recreate that in a man-made way, we should be getting a naturally balanced bacterial environment back. It normalises us and reduces practically all chronic illnesses."

She suggests consuming fermented food at every meal, filling your house with plants and examining carefully everything you put on and in your body.

"Everyone's closing their windows and sticking on an air purifier thinking that it's giving the body a rest," says Buckley. "In fact, it's stressing the body out because it's creating an environment that we're not supposed to live in. Astronauts live in sterile environments and get really sick. Eat more fresh, real food from good sources. Spend money on that rather than air filters. Change your body not the environment. You cannot expect to buy an air purifier and your life will be better. You have to take more responsibility than that."

Westerdahl says that not checking the apps, not worrying about the pollution or simply not seeing it are common coping mechanisms: "If you live in a hostile environment, whether it's infectious disease, or loud noise or other environmental stressors, you have several ways to cope with it. One of the ways is to escape; the other is denial. If you continue to see it as a threat, it's very hard on your body and your mind. The air quality can be very bad but if you've grown up with it, survived and stayed, you probably don't see it that way. Whatever your view of it, it may still harm your health. The absolute best thing you can do at an individual level is to support the Hong Kong government in its regulatory actions to improve the air quality. After all, isn't having clean air more important than being able to buy a face mask or air purifier?"

"I am afraid to say," says Tian, "that I believe that on an individual basis, there is very little we can do about the pollution. Really, at an individual level, the only solution for the pollution, if you are very worried about it or very sensitive to it, is to leave. On a large scale, we can and should deal with the causes. We can use cleaner vehicles, make sure our local power plants are clean and push for cleaner air."

Individuals can demand cleaner air by lobbying both their district councillor and the EPD. Each citizen can also try to reduce their carbon footprint, buy and dispose of less, reuse and recycle. Every new gadget, item of clothing or toy we buy is likely to have been made in a factory across the border. The majority of the energy we consume in Hong Kong comes from coal-powered stations that pump airborne particle matter into the skies. Grass root action can filter up just as public policy can filter down.

"There is room for optimism," says Tian. "We hope and expect that in five to 10 years, the air will improve. Lung cancer doesn't develop over one or even five years. It develops over 20 years. Even for smoking, if you stop smoking, you decrease the risk of lung cancer a lot. If the air improves, the health risks will decrease."

By the same author: Air pollution and our children