A five-minute primer on an issue making headlines I can barely see across Victoria Harbour today. Today? Don't you mean almost every other day this summer? And all last winter, now that you mention it. What's going on? Your gorgeous view is being obscured by air pollution, or as I like to call it by its scientific name, respirable suspended particulates and lots of other muck. The Hong Kong Observatory and the Environmental Protection Department usually explain that the reason for this tropical peasouper is due to the weather conditions. There's either calm air subsiding around a typhoon in the vicinity, or a weak northerly wind prevailing in southern China. Basically, there's just not enough wind around to blow away the pollution. But where does it come from? Face it, Hong Kong and the Pearl River Delta aren't exactly the most environmentally friendly spots on Earth. Greenpeace reckons that 80 per cent of the air pollutants that reach Hong Kong come from southern China. Under very still wind conditions, air pollutants emitted from cars and buses, power stations and factories here and on the mainland have nowhere to go, so they just sit there, obscuring your view, clogging up your nostrils, and getting down your throat. Is it dangerous? Let's put it this way, if you're a Ventolin-inhaling asthma sufferer or you're awaiting a triple bypass, don't go running up any hills if you can't see the top because of the smog. The government advises people with heart or respiratory illnesses to stay inside when the air pollution index reaches very high levels. What constitutes very high? The air pollution index (API) ranges between zero to 500. From 101 to 200 is considered 'very high', while 201 to 500 is 'severe' when symptoms of people with heart or respiratory illnesses can worsen and healthy people are likely to experience eye irritation, wheezing, coughing, phlegm and sore throats. The highest we've had so far this year is 178 on the API for the outlying island of Tap Mun in July. Last year, Tung Chung took a brief peep at the 'severe' side of the scale when it reached 201. In fact, the air quality in Tung Chung has become so bad that 25 per cent of residents say they'd like to move out of the area. Is this nasty air Hong Kong's dirty little secret then? Not any more. The world's press came visiting for the opening of Hong Kong Disneyland last week, which happened to coincide with one of the most polluted days so far this year. How is this pollution measured? The Environmental Protection Department started the air pollution index in summer 1995. It calculates the air pollution level by measuring the key pollutants - nitrogen dioxide, sulfur dioxide, ozone, carbon monoxide and respirable suspended particulates - and whichever one is the highest is reported as the API for that hour. How do we compare to other cities? Campaign group, Clear the Air, says we're not the worst. Shanghai and Beijing still rank way higher in terms of air pollution. However, London, New York, Singapore, Taipei and Tokyo still have quite a way to go before catching up with our filthy air levels. Well, time for a stroll. Luckily, I keep my surgical mask handy. Sorry to tell you, but that mask won't help much. Surgical masks can't filter out tiny suspended particulates nor nitrogen dioxide gases. Oh, and another friendly word of advice - holding your hand in front of your mouth doesn't help either. I might be able to get you a good deal on a gas mask though.