Author: Antonio Da Roza

Date: 24/3/10

On 22 March 2010, Hong Kong’s Air Pollution Index reached its maximum levels as the air quality was severely affected by a sandstorm that had originated from northern China. A sandstorm that originated from northern China was transported southeast as far as Taiwan, before a prevailing easterly wind brought the sandstorm dust to Hong Kong.

Hong Kong's waterfront during the sandstorm episode (22/3/10) - Alex Hofford (

Antonio Da Roza conducted an interview with Professor Wong Tze Wai, co-director of the Centre for Occupational & Environmental Health Studies of the Chinese University Hong Kong's School of Public Health and Primary Care, about the high levels of pollution experienced in Hong Kong on 22/3/10, the Air Pollution Index, and what steps should be taken to protect child health in the event of high levels of air pollution. (1)

1. Professor Wong, what can you tell us about the high levels of air pollution yesterday? Was it related to the sandstorms in Beijing?


Hong Kong's waterfront during the sandstorm episode (22/3/10) - Alex Hofford (

Professor Wong: I am not an expert on meteorology but from exchanges with other academic and professional colleagues, I understand that the present air pollution situation is strongly linked to the sandstorm that originated from northern China and was transported to the southeast region as far south as Taiwan. It appears likely that the prevailing easterly wind since Sunday brought the sandstorm dust to Hong Kong. Evidence of this conclusion is backed by the observation that the air quality monitoring stations in the eastern parts of Hong Kong: Tap Mun, Hong Kong East and Kwun Tong, as well as Shatin and Tai Po, all recorded the highest levels of PM10, which decreased in stations in the west, such as Tsuen Wan, Kwai Chung, Yuen Long and Tung Chung. Generally, air pollutants from industrial activities in the Pearl River Delta affects readings in stations like Yuen Long the most. This is therefore a strong piece of evidence that the incident originated from the sandstorm in northern China.

2. Yesterday, the Air Pollution Index (API) climbed to the maximum of 500. First, can you explain to us what the composition of the air pollution was yesterday?

Professor Wong: If sandstorm dust accounts for most of the PM10, then we should expect the ratio of PM2.5 (smaller particles that are a product of fossil fuel combustion, largely from transport and power plant sources in Hong Kong or the Pearl River Delta) to PM10 to be low. In Hong Kong, the ratio of PM2.5/PM10 is generally around 70%, in other words, PM2.5 accounts for 70% of PM10 measured by weight. If the source of this air pollution episode is indeed the sandstorm, then we would expect the PM2.5/PM10 ratio to be much lower than 70%. According to Prof. Alexis Lau (HKUST), the ratio is 25%. This supports my earlier hypothesis.

3. Secondly, what can you tell us about the health risk associated with the high levels of air pollution yesterday?


Jogging during the sandstorm episode! (22/3/10) - Alex Hofford (

Professor Wong: It is difficult to assess the health impact given the lack of air monitoring data on PM2.5. (which is not routinely recorded in the Government’s Air Quality monitoring stations). However, given the low ratio of PM2.5/PM10, it is reasonable to believe that the overall impact on health is less severe than if we estimate the concentration of PM2.5 based on extrapolation from the PM10 level based on the 70% ratio. Wind-blown dust from sandstorm would be less toxic than PM2.5. Even though the PM10 from the sandstorm is fine enough to penetrate into our lungs, it does not exert the same effect as what researchers including myself earlier reported in our studies of air pollution and the risk of deaths and hospital illnesses of the heart and lung. PM2.5, being made up of highly toxic (and cancer-causing) organic compounds, is much more harmful to health compared to sandstorm dust, that is made up largely of silica and other earth crustal elements. Nevertheless, people exposed to this type of dust will still experience symptoms of the respiratory tract and cause illness, as the particulates will irritate the respiratory tract deep in our lungs.

4. Can you explain to us what it means when the API reaches ‘maximum’?


News report on the API at maximum during the sandstorm episode (22/3/10) - Alex Hofford (

Professor Wong: The API is linked to the highest recorded level of four air pollutants relative to the Air Quality Objectives presently used by Government. The AQO of PM10, for example, is based on the 24-hour average concentration of 180 micrograms per cubic metre. To calculate the API, it is necessary to compare the average concentrations of PM10 over the past 24 hours with this AQO of 180. Hence there is an inevitable delay between the real-time concentration of PM10 and the hourly reported API, which must take the past 24 hour readings into account. Another point to note is that the API is based on set AQOs and the air pollutant that scores the highest of all four, not on the estimated total impact on health of all pollutants. Hence, the API might give a perception that is different from the actual impact on health.

Instead of relying on the API, it is probably more informative to note the hourly recordings of the concentrations of all of the air pollutants monitored. This can be obtained from the Environmental Protection Department website. Other websites also make use of the EPD data to produce their own interpretations. We are currently working with the EPD to review the API and recommend a scientific system that reflects the health risk more clearly and based on local health basis.

5. According to the Government, they recommended when the API reaches ‘severe’, “the general public are advised to reduce exertion and outdoor activities”. What health advice do you think should be given under yesterday’s circumstances?

Professor Wong: The Government recommendation to reduce outdoor physical exercise is sound advice. In addition, children, the elderly and those with heart and lung diseases are advised to stay indoors and refrain from physical exertion. In general, indoor air contains less PM, but we must be careful to reduce or eliminate important indoor sources: cigarette smoking (through abstinence) and gas cooking (through adequate kitchen ventilation and exhaust system over the stove), and harmful chemicals such as formaldehyde and other volatile organic compounds released from furniture and other materials. There is no need to suspend school, or advise workers not to work. We need to note that in many work environments, such as construction sites or other worksites that involve dusty operations, the concentration of respirable particulates can be much higher. These workers should be protected with suitable dust masks or respirators, as a routine occupational health practice. If we need to stay outdoors for a prolonged period and wish to protect ourselves from the sandstorm dust, surgical masks are not effective, but an N95 respirator that fits one’s face will reduce the inhalation of PM10 significantly and will have some beneficial effect – generally, air pollution is a combination of particles and gases, and the gases are not filtered by the mask.