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The Hong Kong AQHI can hide dangerous levels of air pollution

Air pollution levels in Hong Kong regularly exceed safe levels and by some estimates, cause the death of 6300 people per year in the city.

With such dangerous air quality, it is crucial to know when it is safe for outdoor activities, and when we should remain indoors with the windows closed.

All developed countries have an air quality index

To address this and provide timely health advice, all developed countries provide some form of air quality index. Europe and the US each have an Air Quality Index (AQI), Australia uses an Air Quality Category system (AQC) in NSW and Victoria, and in Hong Kong, we have the Air Quality Health Index (AQHI).

The AQHI was launched in 2013 with the stated objective to "provide more timely and useful air pollution information to the public".

Unfortunately, it often fails in its stated goal.

Unlike other countries, the HK AQHI averages pollutants
The Hong Kong AQHI will read Low when PM2.5 pollution is 700% of the WHO maximum level, if gaseous pollutant levels are low.

PM2.5 is very dangerous by itself, leading to heart disease, strokes, cancers, COPD and other health problems. Other pollutants can be just as threatening to health - NO² for example is toxic at levels >200 μg/m³, causing significant inflammation of the airways.

The Hong Kong AQHI averages all of these, resulting in a reading that will under-represent the true risk when individual pollutant levels diverge (as they often do).

The below example highlights the problem, where a PM2.5 level 7 times the upper WHO limit results in a low reading under the Hong Kong AQHI system, and corresponding health advice that no action is required.

This is contrasted with the European AQI, the US AQI, and the Australian AQC, which interpret the exact same pollutant levels as "Extremely Poor", "Very Unhealthy", and "Very Poor" respectively.

Example Scenario

PM2.5 at 700% of the WHO maximum limit (25) gives an AQHI of Low

Pollutant levels (μg/m³):
Hong Kong
European Union
United States
Extremely Poor
Very Unhealthy
Very Poor

No response action is required by anyone.

Everyone should reduce physical activities outdoors.

Sensitive groups should avoid physical activities outdoors altogether.

People with heart or lung disease, older adults, children, and people of lower socio-economic status should avoid all physical activity outdoors.

Everyone else should avoid prolonged or heavy exertion.

General public should AVOID outdoor physical activity....

Sensitive groups should STAY INDOORS as much as possible with windows and doors closed until outdoor air quality is better.

The AQHI under-represents most recent readings

In addition to being the only index compared here that averages over pollutants, the Hong Kong system ignores recent spikes by averaging levels over the past 3-hours.

Gaseous pollutants such as NO² and SO² can have immediate and life-threatening effects on sensitive populations, triggering asthma attacks, hospitalisations and deaths.

The HK AQHI use a 3-hour average level for Ozone, NO² and SO². Combined with a reporting lag of up to 90 minutes, the values provided are often of little use to both sensitive and general populations trying to time their outdoor activities to minimise the health impact.

The European and US AQI standards also suffer from a time-lag problem. They use 24-hour averages for particulate pollution (i.e. PM2.5 and PM10). This is done because the most concerning effects of particulate pollution are long-term, such as heart and lung diseases, strokes and cancer, but it reduces the utility of the index for decisions on daily activities.

To overcome this, the US AQI offers a "Now-Cast" version of its AQI, which weighs recent readings more heavily, but it is still only suited to areas with slowly changing conditions.

In areas such as Hong Kong, where the geography and polllution sources mean that air quality can change rapidly, it is vital to monitor the most recent pollutant concentrations.

It's time for an accurate pollution index in Hong Kong
Most recent methodology update 2013 2017 2015 2020
Rates pollutants individually (i.e. not average)
Uses 1-hr levels for pollutants with accute affects
Uses 1-hr levels for particulates *

* Adjusted AQI

The US AQI based on latest particulate levels provides accurate and timely advice

We recommend using the US AQI, adjusted to use the latest readings for PM2.5/PM10. The system does not average across pollutants and provides a reading that is based on the most recent 1-hour pollution data. Accompanying health advice is clear and actionable.

Embee publishes the current AQI based on this system (in addition to the HK AQHI) for each of Hong Kong's 18 monitoring stations and over 1000 school locations, along with health advice and individual pollutant charts, all updated hourly.

You'll find all of the information here:

The Embee team.

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