City University of Hong Kong

Air pollution in Hong Kong—how bad is it and where does it come from?

7 October 2016

Written by Dr Zhi Ning, Assistant Professor, School of Energy and Environment, City University of Hong Kong

Air pollution in Hong Kong—how bad is it and where does it come from?

Almost every week, we read with horror about how bad air pollution levels are in Hong Kong and the harm they cause to the health of city dwellers. It is even driving big companies out of the city to places like Singapore in search of better and safer air for the employees and their families. Fine particulate matter (airborne particles of 2.5 micron diameter or smaller) and ozone are the worst culprits.

Some of these particles are emitted directly by human activities while others are formed in the atmosphere through chemical processes. Ozone is invariably produced as a product of reactions between hydrocarbons and oxides of nitrogen reacting in the presence of sunlight.

So, how bad is it here and what can we do about it? We have our own way of communicating the healthiness of air: the Air Quality Health Index. It ranges from 1 (low) on a really good day to 10+ when the problem is serious. Air quality is usually driven by fine particle concentrations and it is these that are responsible for the most adverse health consequence, while ozone, nitrogen dioxide and sulfur dioxide occasionally make significant contributions to the index.

When air quality worsens and the index is high, some people face a heightened risk of death. In Hong Kong, we reach such levels of 10 or 10+ levels several times a year. What factors contribute to these very bad days?

Firstly, Hong Kong is a unique place with high density of tall buildings and inhabitants. We have many vehicles that contribute to the harmful emissions that become trapped in the street canyons in between buildings. This gives rise to localized hot spots where particle and nitrogen oxide concentrations are high. Being a major seaport also means that sulfur dioxide emissions from ships adds to the toxic concoction city dwellers inhale every few seconds.

Secondly, although Hong Kong does not have any major industries that still belch organic fumes or coal burning power plants that release smoke or even much biomass combustion, many of these pollutants are imported on a massive scale from just across the border. This pollution is periodically transported southward, especially in winter and during typhoon periods, denying us high visibility and healthy air.

Simply put, reducing local emissions in Hong Kong will help, but it is not sufficient. However, our transport system has not been properly routed to avoid the formation of hot spots in street canyons. At the same time, poor urban planning has led Hong Kong to become a city with so many calm (stagnant) areas for the pollutants to disperse effectively.

This we ourselves are to blame for. However purely local solutions are limited, so we must expend effort in collaborative pollution control across the Pearl River Delta. Improved relations with bordering authorities is on-going, but it is not clear how effective it is in fending off regional air pollution.

A range of reasons explain why Hong Kong has failed to achieve the levels of air quality expected of a modern city.  As there is no single source to this air pollution problem, improving air quality needs to find a range of solutions and these need to be pursued with enthusiasm and tenacity.