This year, Sasha Haldane and her husband, Will Hayward, took one of the biggest decisions of their lives. They decided that, for the sake of their two children, they would leave Hong Kong. Haldane grew up in the city and, apart from a brief spell in Australia and New Zealand, had lived and worked here all her life. Max, 10, and Alyssa, six, were both born and raised in Hong Kong. Their parents had good jobs and the children were settled happily in schools. One factor alone compelled them to leave. "Will became very concerned about the air pollution as he is an ultra-marathoner," says Haldane. "He was really noticing the air quality when he was running, and I was noticing that you could no longer see across the harbour most days in winter. What was meant to be the best time of year, October/November, was now the worst." Hayward worked as head of the psychology department at the University of Hong Kong. Concerned about the deteriorating air quality, he sought out Professor Anthony Hedley, an expert on air pollution at HKU's School of Public Health. He discussed the problem with Hedley and set about reading a long list of scientific papers on the effects of pollution. "As a result of those papers, we realised that if we did not leave, we were going to reduce our children's life expectancy. What we understood was that, whilst it was bad for younger children living in this air quality, the most impact was going to come when they were teenagers, when the greatest expansion of their lung growth was occurring. The pollution could affect that expansion so that when they were much older, in later life, they would be less able to recover from respiratory illnesses and more likely to die earlier." So, with heavy hearts, the family looked for places with cleaner air in which to live. "When a job came up in Auckland, New Zealand, Will was happy to move," Haldane says, from her new home. "He is from New Zealand and has family here. Auckland has very good air quality for the size of the population. But there were no other factors. We both had very good jobs, great friends and a wonderful lifestyle. In January, we moved." Hong Kong has long prided itself on its high life-expectancy figures. However, those living to a ripe old age now grew up with much cleaner air. Lai Hak-kan is a research assistant professor at HKU's School of Public Health. He studies the impact of pollution on health and has no doubt that children growing up in Hong Kong now should expect shortened lives. "Our studies find that 10mg of particulate matter will cause two years of life expectancy lost. In Hong Kong, we are looking at 45mg to 50mg of such matter. Theoretically, this means 10 years of life lost." The World Health Organisation recently reported that one in eight deaths globally is linked to air pollution, making it "the world's single largest environmental health risk". Out of the seven million deaths attributed to pollution in 2012, six million occurred in Southeast Asia and the Western Pacific - the region containing Greater China. One of the biggest predictors of life expectancy is lung function. Our lungs take in clean, oxygenated air and distribute it to our tissues to enable them to function. When we breathe in dirty air, our respiratory system - including its major organs, the lungs - becomes damaged and cannot operate properly. "There are a number of negative health impacts for air pollutants but most of the findings from medical literature indicate that the main impacts are on cardiovascular health and also on respiratory diseases," says Lai. "Some studies also indicate that longer-term exposure can be linked to cancer development." The impact is worse in children than in adults. "One of the basic principles we know when we are handling health problems in children is that they have small organs which are growing day by day. So, all those cardiovascular, respiratory and lung functions need to be developed into maturity in health to prepare them for the demands of the future. If, however, they are exposed to things which are harmful, the effect is much more significant than if these systems are already fully developed. The implications are much higher and have a lifelong consequence. We have growing evidence that long-term respiratory and cardiovascular problems are strongly related to early exposure to environmental pollutants. The lungs and heart are immature in particular before primary school." Watch: How to deal with Hong Kong's smog Hong Kong's once fragrant harbour is thick with a fug of cargo-ship emissions, traffic fumes and output from coal-powered power stations and factories in Guangdong province. The city's topography of mountains and high-rise buildings causes a canyoning effect, which prevents toxic air from dispersing. All this contributes to the smog that Haldane had noticed folding its way round the city, billowing through the harbour and hanging between buildings. This smog consists of many substances. First is sulphur dioxide (SO2), an odourless gas that is caused by the combustion of fossil fuels in power plants, industrial boilers and vehicles on land and at sea. SO2 damages the respiratory system and aggravates respiratory disease, for example bronchitis. It causes wheezing, shortness of breath and coughing. It can also cause cardiovascular disease in people with sensitive respiratory systems, including asthmatics, the elderly and children. Second, nitrogen oxides (NOX) and nitrogen dioxide, which are formed through combustion at a high temperature in power plants and vehicle engines, at land and at sea. Nitrogen dioxide is a smelly brown gas that irritates the lungs and lowers the body's resistance to respiratory infections such as the flu. People with respiratory problems (for example, asthmatics) are more susceptible to the effects of NOX. In young children, these gases impair lung development and chronic exposure leads to structural changes in the lungs. Third, ozone, which is formed by the reaction of nitrogen dioxide in ultraviolet light and makes a photochemical smog. Ozone irritates mucous membranes in the nose, throat and airways. Because it's "consumed" by the gaseous pollutants found by roadsides, ozone is often more prevalent at higher levels in outlying areas, such as the New Territories, where people may feel there is less pollution. It causes coughs, chest pain, throat pain and eye irritation, decreases lung function and inflames the respiratory system. Those are the main gases. Then come some non-gaseous nasties - fine particles called respirable suspended particulates (RSPs). They are made of matter that is suspended in the air but is not a gas - so liquids, solids or a mixture of the two. Measured by their size, RSPs are divided into two categories: PM10 and PM2.5. The PM10 particles measure 10 micrometres or less across and PM2.5 particles measure 2.5 micrometres or less. These miniscule particles are produced by combustion - particularly in diesel vehicles and power plants - construction, incineration and, in nature, the sea. Often, the particulate matter is small bits of unburnt or partially burnt carbon attached to elements such as silicon, aluminium, sodium, calcium, manganese, iron, barium, mercury and lead. Lead is particularly dangerous to young children as it damages the nervous system, red blood cells and kidneys and can cause brain damage. Many RSPs are carcinogenic. The problem with RSPs is both their size and the chemicals they contain. PM2.5 particles are so small that our noses can't filter them out. They get breathed straight into the lungs, where they deposit on the lining and have no way out. RSPs cause a wide range of problems, such as blocked airways, coughs, decreased lung function, aggravated asthma and the development of diseases such as bronchitis. In people with existing heart and lung disease, they can cause premature death from the likes of lung cancer. Dr Lee So-lun is honorary clinical associate professor at HKU's Department of Paediatrics and Adolescent Medicine. She explains why children are so much more vulnerable to these pollutants. "Firstly, it's biological, meaning that the build-up of their body is different. All the organs are still growing and maturing during childhood. Any insult to growing organs causes a long-term impact, unlike in adults, who have mature systems. Plus, their body system cannot remove toxins as well as adults. Then we have to look at the lungs because that is where the air pollutants are going. Lungs are maturing in childhood and we know that toxins on growing lungs are harmful. Our immune system is also immature in childhood. So, if you have a toxin that interacts with a genetic predisposition, it can cause things like allergies and asthma. "There are also physiological differences," Lee continues. "The lung wall in a baby is proportionally much larger than an adult's. This means far more pollution circulates through the system. Babies and small children also have tiny airways, so anything that aggravates them will have a far greater impact. Also, children are much more prone to mouth breathing, which stops their noses filtering dirty air. Children are usually much more active than adults, so they tend to run around breathing in much more polluted air." Scientific studies show that mothers living in polluted areas are at greater risk of having babies with low birth weights and congenital malformation. Animal studies have linked attention deficit hyperactivity disorder (ADHD) and hyperactivity to early exposure to air pollution. Rebekah and Stirling Brinner, an IT manager, lived in Hong Kong for five years. Both of their children, Zoe, now five, and Tabby, two, were born in Hong Kong. They struggled with Zoe's reaction to the pollution. "Zoe was born with eczema," says Rebekah Brinner, a former nurse. "But conditions in Hong Kong exacerbated it. As your skin is your largest barrier to infection, she got minor illnesses more often than her peers, and she had trouble sleeping due to constant intractable itch. The worst came in the form of repeated skin infections. Her whole body was covered in painful, itchy, oozing sores. She was absolutely miserable and was hospitalised with a major infection at one point, not to mention the frequent trips to private and public doctors for outpatient treatment." The Brinners ruled out other possible causes such as soaps and allergies. Their cure came whenever they left Hong Kong on holiday. Almost immediately, Zoe's infections and skin flares subsided. As soon as they returned to the city, her skin flared up again. "I know that it's hard to pinpoint the specific cause of eczema, as it's at least partially genetic and, if you have it, it never actually goes away, but it can be managed and you can reduce triggers. Every doctor we've discussed it with has agreed that the pollution was a likely factor, especially considering that we ruled out the other likely triggers. "We had to leave because our daughter had a very low quality of life. She couldn't be allowed out of my sight otherwise she would scratch until she bled, which could and often would lead to another infection, not to mention potential for scarring and need for more strong medications. Stirling and I discussed it and decided we needed to get her out of the toxic environment as soon as possible, and he was lucky enough to quickly find a job." They left Hong Kong in May last year. "We now live in a suburb of Philadelphia, in the United States. We've been here nearly 11 months and she hasn't had a single infection or flare. We've significantly reduced both the potency and frequency of topical steroids and she has gained independence because she can be allowed to do things on her own. She sleeps better and has gone from the third percentile on the Child Development Centre growth chart up to the 25th percentile." The Brinners miss Hong Kong culture and friends but are certain they made the right choice. "Zoe asks frequently when we are moving back but she is thriving here. We've had some rough days, but then she'll say that her skin doesn't hurt anymore. So far, the future looks bright, our daughters are excelling. Ultimately, Hong Kong seems unable to do much about the pollution as so much of it comes from the mainland. Sad, as we really grew to love the city." Forty-seven per cent of Hong Kong's pollution comes from the Pearl River Delta. The rest is made up of the city's own roadside, marine and power-station emissions. Global studies show that the highest impact on children's health is from roadside pollution. Living, walking or going to school next to a busy road is much more likely to harm children than ambient air pollution from the mainland. Arthur Lau Pui-sang is adjunct associate professor in the Division of Environment at the Hong Kong University of Science and Technology. In collaboration with Chinese University, he is running a citywide study of the health effects of pollution in primary schools. The study looks at 3,000 students aged nine and 10 in 30 primary schools to find out how the air quality is affecting them now and consider their long-term pulmonary development. The study will follow up with these children in two years and then, funding permitting, a further two years later. Lau's initial findings show that the distance from a road is a major factor in child health. A safe distance is defined as 200 metres. The direction the windows face and general wind direction also dictate how much pollution enters a school environment. As does the proximity of school buses, which often have polluting engines left idling. The season is important, too. In summer, when the windows are shut and the air-conditioning is on, the air inside a classroom is much cleaner. In winter, when windows are more likely to be open, classroom air quality is dependent on outdoor air quality. "The reality in Hong Kong is that there are lots of schools next to busy streets," says Lau. The WHO has drawn up a gold standard of air-quality objectives (AQO) to provide a guideline about safe levels of pollution. As one would expect, Hong Kong falls far short of the WHO's objectives. As a result, the government has set up some interim objectives of its own. The city uses the WHO's guidelines for nitrogen dioxide and carbon monoxide but has adopted the lowest interim target levels suggested by the organisation for SO2, ozone and PM2.5 particles, with a slightly better target for PM10 particles. Air is measured in 12 general stations and three roadside stations throughout Hong Kong. This information is compiled into the Air Quality Health Index (AQHI), which is available online and via an app so the public can view levels of pollution and decide how best to protect their families on bad days. The system looks at levels of the four major pollutants and works out in real time what the percentage risk to health is. It then categorises the risk on a scale of one to 10. At times and in places when/where a reading is high (7+), the government's advice is that children reduce the amount of time they spend - and especially exert themselves - outside. Once it hits very high (8+), children are advised to reduce outdoor time to a minimum, and when it's serious (10+), to stay inside. In effect, that is the government's advice - go outside less or stay indoors. Under the previous air-pollution index, which was replaced by AQHI at the end of last year, 6 per cent of days were considered dangerous to health in Hong Kong. Under the new system, 30 per cent of days are considered dangerous. If the index conformed to WHO standards, about 90 per cent of days would be deemed dangerous. Wong Tze-wai is a research professor at Chinese University's School of Public Health and Primary Care. He led the project to develop the AQHI system and is a specialist in community medicine who has conducted research on air pollution and health for more than 20 years. "For a warning system to work," says Wong, "you need to avoid the phenomenon called 'information fatigue'. If we issue a warning to the public every time the air-pollutant concentration [for example, PM2.5] is above the WHO 24-hour guideline [25 micrograms per cubic metre], we end up overwarning, because on most days, Hong Kong's level of PM2.5 is around 30 or so. If we overwarn, the public will not respond. "Moreover, it would not be desirable to advise the schools to cancel the PE class every time any one air pollutant exceeds the WHO guidelines, for that would mean no PE classes on 90 per cent of days in a year." So, if the only available solution to bad air days is to stay indoors, how much cleaner is indoor air? "In general, indoor air quality is better than outdoor air quality, provided there is no indoor source of air pollution, for example cigarette smoking, incense burning, gas cooking, furniture - especially new - that emits harmful volatile organic chemicals such as formaldehyde," says Wong. Lee warns parents not to become complacent just because a child doesn't show any immediate symptoms. "We are looking at this from a population level. Just because your own child doesn't seem to be affected does not mean it's not a big issue. It is a serious medical concern and something we have to deal with because of the long-term effects. "The weather, outdoor environment and indoor environment all affect us," says Lee. "Outdoors there's humidity and pollen, for example. Indoors, house dust mites and passive smoking. There are so many variables in the environment. While one element of the environment is high at any given time, others are low and that is why people can react differently." Undersecretary for the Environment Christine Loh Kung-wai, an environmentalist and herself a mother, is committed to reducing the city's pollution and, since she began her tenure, Hong Kong has introduced measures to tackle the problem. Loh recognises that both a local and a regional approach are needed and has been working with the governments here and in Guangdong to facilitate change. By 2019, Hong Kong will have phased out its most polluting older diesel vehicles and buses. A subsidy scheme is in place to assist owners of taxis and light buses to replace worn-out catalytic converters by the end of this year. Funding is also available for franchised bus companies to clean up their existing fleet. At sea, domestic and river vessels refuelling in Hong Kong had to switch to very low sulphur diesel by April 1 and regulations are being drafted to compel ocean-going vessels to switch to low sulphur fuel while berthing in the city's waters from next year. That would make Hong Kong the first port outside the US and European Union that legislates for a fuel switch at berth. By 2017, Guangdong is aiming to reduce PM2.5 levels by 15 per cent. Furthermore, Hong Kong and Guangdong have already successfully completed one round of pollutant reductions. "In 2012, we agreed on a second round of targets for 2015 and 2020," says Loh. Her goal is to meet the city's current air-quality objectives by 2020. She acknowledges that these objectives don't meet the WHO's standards but explains that interim goals are necessary as there is no point setting standards that simply cannot be met. She promises the objectives will be reviewed - and tightened if possible - every five years. "Probably more than anyone else, I know how much we are doing in Hong Kong and in Guangdong. We must implement our plans well and then we must do more still," says Loh. "Five years ago, I was the CEO of a non-profit think tank where we did surveys to assess the level of concern in Hong Kong about air quality. Today, there is no question in government that air pollution isn't a top priority. The chief executive has already made this clear." "The situation has actually improved a bit in the past decade," says Dr Alfred Tam Yat-cheung, a paediatrician in private practice and a member of the Advisory Council on the Environment. "The general stations have recorded decreasing levels of pollutants except for O3 while the roadside stations have recorded increasing levels of NOX and RSP. So although the general situation has improved, the roadside situation has not. "I think it is safe to bring up children here as long as you avoid the roads," he says. "The general situation is better than many cities in the mainland and it [is getting] better slowly." Too slowly for people such as Haldane, though. Unlike most parents in Hong Kong who fear for the well-being of their children, she could leave the city without too much trouble, but she'd prefer to return. That will not happen, she says, until the government adopts the WHO's AQO. "I realise that Hong Kong is hampered in terms of the pollution from China but the government could crack down more on the pollution from incoming ships and the traffic, and should have done so years ago instead of listening to special interest sectors. "I was not happy to leave. I am a Hong Kong girl. I have my entire life there. I love Hong Kong. It is my home. Right now, I am really homesick. But I had to make the decision for my children. I had to give them the best chance for a long life." For an alternative look at what Hong Kong's monitoring stations are telling us about air quality, take a look at HKU's Hedley Environmental Index hedleyindex.sph.hku.hk/html/en .