Hong Kong's air is often polluted. However, it’s bewildering knowing what the air quality index means, how the different scales compare and what it means in terms of altering our daily schedules. Often information is written too scientifically, too abstract and just way too many words. I’m going to cut through all that and write in plain English with no whittering and warbling. I’ll explain what the numbers mean. I’ll cover the health impacts of this invisible killer. I’ll help you make an informed decision on whether your kids should be taken to the outside playground today or switch it to a playdate at home. You’ll be able to decide for yourself when to turn on / turn up your air purifier and when to flush the rooms clean by opening the windows. As individuals, we don’t have control over the pollution itself but we can certainly control how much exposure our family has to it, without having an unrealistic impact on our busy lives.
The air is full of harmful particles not visible to the human eye. These pollutants are referred to as Particular Matter (PM) and come in a variety of sizes and forms. These particles fall into two categories:
- coarse particles referred to as PM10 are under 10 microns in diameter and
- fine particles or PM2.5 are less than 2.5 microns in diameter.
What is PM2.5?
PM2.5 particles are at least 30 times thinner than a human hair. Due to their size and weight, these particles linger in the air and travel huge distances with the wind. As an illustration, PM2.5 particles that originated in Taipei could feasibly be found in Hong Kong when the wind is blowing in the right direction. Now draw an arc from Taipei centered on Hong Kong and think about how many power plants, factories and diesel trucks fall within that area.
Hong Kong generally has two pollution seasons. In the Spring/Summer, the wind comes from the east where there is nothing but sea. The air picks up water, leading to humid but cleaner air. In the Autumn/Winter, the air sweeps down from the north or north west through China bringing pollution into Hong Kong. In addition, Hong Kong is exposed year round to diesel burning container ships emitting some of the deadliest PM2.5 particles. Even though each individual particle is invisible to the human eye, in aggregate, they can reduce visibility forming a haze on particularly polluted days.
Where do PM2.5 pollutants come from?
Fine particles are usually released into the air when things are burnt. Examples are power plants, automobile and maritime engines, forest fires (such as those recently in Indonesia) and factories.
Why is PM2.5 an invisible killer?
Particulates are the deadliest form of air pollution. The WHO has designated them as a Group 1 carcinogen which means that there is sufficient evidence that they can cause cancer in humans.
Our body responds to particulates by producing mucus to trap the particles. The contaminated mucus is then moved out of the lungs up the throat. Ever wondered why smokers and people from polluted countries are always coughing and spitting?
Larger particles can often be trapped in mucus but fine particles travel deep into the lungs. Some of these fine particles (such as those emitted from diesel engines) can even pass through the lung-blood barrier unfiltered, causing DNA mutations, heart attacks and premature deaths. We can’t see them or smell them - that is why we call them the invisible killer. Think twice before going for that run or taking your kids out scooting on a heavily polluted day!
How is it measured?
PM2.5 is measured as a concentration in micrograms per cubic meter or μg/m3. To measure PM2.5, a sample of air in a meter long cube needs to be taken. All the particles inside are sorted and those over 2.5 microns in diameter are discarded. The remaining particles are very carefully weighed. Not very practical, huh?!
Most instrumentation estimates PM2.5 by either counting the particles passing through a beam of light or by measuring the light scatter caused by particles passing through the light. The reading is an estimate because the instrument doesn’t weigh each particle. An assumption is made instead. I have found that these instruments are effective to varying degrees. Over the best part of a year, I have been testing various technologies from different manufacturers, and have found that the most important factor for accuracy is the type of light source. I have found that devices featuring well calibrated laser sensors can be consistently accurate compared with those featuring infrared sensors.
How can we minimise exposure?
Fine particles generally take a while to seep into your home when the windows are closed. I make sure that my windows are closed when the air is polluted and opened to flush fresh air through my home when it's clean. Make sure that your windows aren't leaky. If they are, many hardware stores sell draft excluders that can be easily taped to the outside of the window frame to give a better seal. We also have draft excluders fitted to our front and back doors.
In heavy pollution, I change outdoor playdates for indoor ones and my husband goes to the gym rather than running outside.
Peek Concepts has no association with any of the companies selling HEPA filters but through my experimentation, I have found that they are very effective. They are an absolute must for my family. I have two brands in my household – the pricey IQ Air (HK$14,800 from bumps to babes) in the bedrooms and the more affordable Sharp (HK$2980 from Fortress) in the living areas. My air quality monitor found PM2.5 concentrations of 0 μg/m3 at the outlet of the IQ Air versus 0.3 μg/m3 (excellent air quality) for the Sharp. The IQ Air reduces PM2.5 concentration faster than the Sharp in similar size rooms. There are many other brands providing HEPA filters with varying prices and performance. It’s personal opinion as to how much extra you pay for the incremental effectiveness. My message is that there are effective air filters available at a range of price points - so keep your family safe.
Check your air filters with a PEEKair, the latest PM2.5 air pollution monitor designed by my company for Hong Kong households. I move it from room to room, taking real-time readings of the pollution levels in our home. I can then quickly decide whether our air filters need to be turned up or down. When PEEKair shows that the air is clean, I turn our filters off, keeping my husband happy by reducing electricity bills and extending the life of our HEPA filters.
Visit the PEEKair product page after you finish this blog post to learn more and to read the real-life reviews from our customers.
Beyond PM2.5, we grow a range of air filtering plants in our household but more on that in a future blog post.
My blog on Where to buy air filtering plants is now available.
How do I know what the outdoor PM2.5 levels are and what do they mean?
I use two apps.
- HK AQHI is the Hong Kong Government’s app. The Government has monitoring stations across Hong Kong (none on the Southside, unfortunately). As I'm generally intersted in PM2.5, I disregard the overall headline air quality health index level but go straight into the detail. Select a monitoring station and press the "i" next to it. Then scroll to the bottom of the pollutants page and you will see the PM2.5 concentrations in μg/m3. Here are some snapshots on how to get there.
I also use a website called AQICN.org. Bookmark it on your phone. The data comes from the same Government monitoring stations as HK AQHI so why do I use it?
- It singles out the PM2.5 reading making it convenient
- It can give an overall average for the Hong Kong region or set it to a specific station
- The reading is presented in a user friendly way. The bars indicate the past 48 hour trend and are coloured in accordance with the US EPA’s categorisation of pollution levels
- It collects feeds from monitoring stations worldwide so I use it on holiday
- Sometimes I'm a tad skeptical because it looks polluted outside, yet the official PM2.5 readings from the apps are low. Scroll down to look at the difference between temperature and dew point. If these are close, be reassured that the murkiness is likely to be fog or mist. My husband learnt that during his pilot training.
One confusion that people often have, is that the numbers from the two apps are different (39 versus 9). This is because they are displayed in different units. The HK AQHI app reports PM2.5 concentration in μg/m3 whereas the AQICN.org rebases the concentration into AQI (Air Quality Index). The latter makes it easier for the layperson to determine whether the air is good or bad. To convert between the two, AirNow.gov in the US has a useful tool on its website:
Convert μg/m3 to AQI: http://www.airnow.gov/index.cfm?action=resources.conc_aqi_calc
Convert AQI to μg/m3: http://www.airnow.gov/index.cfm?action=resources.aqi_conc_calc
The screenshot below shows an example. Here I enter the AQI of 39 from the AQICN example above. You can see the result is 9.4 μg/m3 matching the reading from the HK AQHI app.
Now I know the numbers, what should I do?
Here is a very useful table sourced from the US EPA that my family use. I've added a column to include PM2.5 concentration in μg/m3.
You can download the above guide in pdf format here.
So there you have it. You now know how I make decisions based on PM2.5.
If you liked what you just read, share it with your friends with one click using the icons at the bottom, then sign up to my mailing list to have future blog posts and exclusive offers delivered direct to your inbox.Sources and where you can find out more:
This blog post should not be taken as medical advice. It is intended to be an informative illustration of how my family makes decisions based on PM2.5. Please contact your doctor for medical advice.