After much speculation about how much of the responsibility for Hong Kong's poor air quality belongs to local sources and how much across the border, we now broadly understand.
In sheer volume, roughly two thirds is 'imported' and the rest is emitted locally. If we are to ever have even moderately clean air, much more must be done to reduce the appalling pollution emitted in nearby Guangdong province.
Yet we also now know that for about half the days in a year, generally when the air quality is not at its worst, the pollution comes mostly from local sources. It is only because the bad days, when imported sources dominate, are so bad that the total volume of imported air pollution swamps the local sources for the year as a whole.
It might seem that since the local pollution sources tend to dominate days when the air quality is not 'high' according to way the government measures the air pollution index, that reducing local sources is not a pressing matter. Unfortunately, the current index gives a false sense of how safe the air is. A low to moderate pollution level here would be, by the standards used in other high-income economies, 'high' with appropriate public warnings issued.
The government has taken important steps to curb local pollution. Levels of nitrogen oxides and large particulates are down somewhat. All pollutants, including those which have continued to rise, such as fine particulates, ozone and sulfur dioxide, are lower than they otherwise would have been.
Yet, the fact remains that our air is unhealthy and much stronger actions to reduce local sources will reduce the major health risk each of us face. Such actions would be noticed across the border and help bolster the government's arguments for Guangdong to do more.
Aggressive action locally might also reassure some of those high-flying financial services sector types that Chief Executive Donald Tsang Yam-kuen seems so anxious to convince that if they come to Hong Kong the air will be safe to breathe.
Bill Barron, Hong Kong University of Science and Technology