又一次冬天，又来一次PM 2.5来袭！面临北京冬天的PM 2.5，我开始想：冬天PM 2.5到底比夏天差多少？
我分析了北京美国大使馆近7年的PM 2.5数据，结果夏天平均PM 2.5为92微克，冬天为111微克。
这有多严重？世界卫生组织PM 2.5的24小时上限是25微克。那么北京夏天平均PM 2.5是上限的三倍多，冬天是四倍多。
据英国《卫报》, 乌兰巴托是世界大都市空气污染最严重第二。 2008-2011年，乌兰巴托PM 2.5平均值接近150微克；北京2014年平均PM 2.5是98微克；世界卫生组织24小时上限是25微克。
欢迎加入我们DIY沙龙，了解如何用简单的DIY空气净化器减少家里的有害颗粒污染。Smart Air创始人Thomas Talhelm将解释非常贵的空气净化器的工作原理及如何利用同样的道理保护自己和保护自己的钱包。参加者在现场制作自己的DIY空气净化器带回家。
数据显示很多人的直觉（包括我自己）是错的：PM 2.5在夜间的时候最高（上海除外）。在五个城市，PM 2.5是下午最低。所以如果要安排野餐或者室外活动，一般来说最好安排在下午或者凌晨。
大炮至少净化30.5m2的房间。这个比 3,6000元的Blue Air 203适用面积（22.3m2）要大。
I’ve posted data before showing that outdoor air quality is strongly correlated with indoor particle counts (r = .71), but Chinese New Year gives nerds like me a great chance to see what happens when we get a momentary shock to air quality.
The media made a big deal about people cutting back on fireworks this year out of a concern for air quality, and that may be true, but you can still see a strong spike in PM 2.5 as Beijingers rang in the year of the horse:
Not all that surprising. But what’s more interesting is that you can see a corresponding increase in the particle counts in my collaborator Gus’s bedroom (blue line):
These indoor counts are without a purifier running, so they demonstrate how quickly outdoor air pollution can find its way indoors and how variable indoor air quality can be in a single room over time. Simply put: the worse the air is outside, the worse it is inside.
Yet the glass half empty can also be half full. When outdoor particulate goes down, indoor particulate can go down in a hurry. Check out what happened after the firework apocalypse ended:
A note for fellow nerds: The indoor particle counts are not precisely on the hours, so the apparent time lag between indoor and outdoor counts may be exaggerated.
I recently had a conversation in Beijing that went something like this:
Friend: I’m not sure if I can make badminton tomorrow. I have a basketball game in the day.
Me: Oh man, do you play outside?
Friend: Nah, it’s inside.
Me: Oh, phew. Good.
Friend: Wait, why do you say that?
Me: Oh, the air is way worse outside. I used to feel like I had asthma after playing basketball outside.
Friend: Really? No, they’re not that much different. I saw it’s just 20% different.
Seeing as how nerds cannot let matters of fact go, I started using my particle counter to take measurements of inside and outside air at different locations around Beijing. This answer is important: it tells you if it’s any safer to exercise indoors and how much damage you’re doing to your lungs by choosing that seat outdoors at your favorite cafe or restaurant.
So I took measurements in six locations around Beijing, in apartments, cafes, and my gym. I only chose bad days (pollution concentration above the WHO standard of 25), and I avoided days where it rained (because rain can cause quick changes in air quality). Here’s what I found:
On average, indoor air had only 36% of the pollution outdoors.
Things were a little worse for the smaller .5 micron particles, but still much better than outside:
On average, indoor air had only 51% of the .5 micron particulates of outside air. My guess is that the .5 micron data was worse than 2.5 micron data because it’s easier for smaller particles to get into your home and stay suspended in the air.
There is a lot of variation between places. For the 2.5 micron particles, the locations varied from 14% to 58%. Dr. Saint Cyr also found significant variation between two apartments he lived in, 50% to 70%.
Conclusion: In terms of particulate pollution, you’re safer snagging an indoor seat and working out indoors, particularly on bad days (I’ve seen some argue that we are particularly vulnerable when we work out because we breathe more deeply than normal).
But remember that doesn’t mean indoor air is safe, just better than outside. For example, if your air at home had 40% of Beijing’s concentration last night at 11pm (8/15), you would’ve had 64 g/m3 in your home, which is more than twice the WHO standard of 25.
As usual, I’m posting more on my methods and raw data below.
Dr. Saint Cyr recently pointed me to great tests of air purifiers from the Shanghai Consumer Protection Bureau:
The reason this type of research is so badly needed is that Western research (like this report from Consumer Reports) focuses on allergens, not general industrial air pollution. Allergens are probably more relevant for most Americans, but for those of us living in China, particulate air pollution is the real problem. And a lot of smog is smaller than pollen:
Thus, I was excited to see research that focused on PM 2.5 here in China (results here, in Chinese). In removing PM 2.5, 17 out of 22 models removed more than 90% of PM 2.5 in just 20 minutes in a 30-square-meter room. Pretty good!
Yet even the cheapest of the “non-famous” brands cost about 1,500 RMB. For that price, you could make 9 of my DIY filters, and that’s before eating the cost of proprietary replacement filters (US$200 a piece at IQ Air). Even with these cheaper brands, I still think consumers lack a truly affordable way to get clean air. Using the consumer bureau’s results, I calculated how much you’re paying for each percent reduction in PM 2.5.
Now, their tests weren’t perfect. The biggest downside I see is that they used cigarette smoke as the PM 2.5 source, rather than outside air. It’s not clear to me if cigarette smoke behaves the same as smog in filters. Also, many filters were less effective at removing the formaldehyde that they released in the room. That said, I don’t know how much formaldehyde is in the air normally.
But these results from Dr. Saint Cyr do use ambient air pollution in China (as do mine 1, 2). In tests with doors closed and the filters at their highest settings, Dr. Saint Cyr’s two Alen Air filters, Blueair, and IQ Air all got rid of at least 95% of PM .3. But the Alen Air A375 will set you back US$500 or RMB 5,000 imported in China. Here’s how to get the same results for 166 RMB.
I think you can break the question of whether an air purifier works down into two questions.
Answering this question is more difficult because you need a controlled environment (you can’t open and close windows during the test), and you need to test the air for a longer period of time. Fortunately for you, I’m a nerd, and I’ve been doing these tests for fun for the past few weeks.
According to my particle counter, here’s what the filter did in an hour:
I’ve also tested the effect by running tests with the particle counter on hourly mode. These tests show that the downtrend continues over several hours:
(Note that the hourly test and minute test are on different days.)
The particle counter also gives data on 0.5 microns – even smaller particles. Here’s what that looks like over eight hours:
Conclusion: The DIY purifier works. You can get clean air for 166 RMB, as opposed to 8,000 RMB as long as you know that a HEPA filter is all you really need to fight particulate air pollution in China.
From my perspective, filter companies like IQ Air are taking advantage of how little we know about air pollution and the fact that you need expert instruments to tell whether the filter is working or not. When consumers don’t know how to assess the products we buy, we often use price to tell us whether the product is good. That happens with expensive wines all the time. I’m convinced you can breathe safe air in China for far less than filter companies want you to believe.
Finally, for data nerds like me, I’m including more details on the tests here:
The test above was done starting at 11:30 pm (I’m a night owl) on 6/16/2013, when the outside AQI in Beijing was 230 according to the US embassy’s AQI Twitter feed. (The outside air improved the next day, but results were similar on a later test where AQI actually went up slightly from 195 to 202 during the test. Details to follow here.)
According to comparisons of my particle counter’s tests of outside to US embassy AQIs, an AQI of 230 would convert to about 2,650 on the 2.5 micron count on my reader. (Remember, the particle counter gives the raw number of particles 2.5 micrometers and above per .01 cubic feet. The US embassy takes mg/m3 and converts that to an AQI. Therefore, the numbers are different, although they correlate.)
I did the test in my bedroom with the doors and windows closed. The room is 13.5 meters squared, with two windows.
Astute readers have asked whether I let the particle counter run a bit to get a stable reading before turning on the filter. The particle counter tends to take a 5-10 minutes to get stable readings. To be conservative, I gave it about an hour:
(Update: This question is answered with much more data in my new tests where I left the particle counter on for 6 days, and I turned the DIY Cannon on a timer to turn on every day for 2 hours.)
The spike at the top was when I entered the room to turn the air purifier on and reset the machine, so it may just be noise or it may be the dust I kicked up by walking around. A more stable reading for that time would probably be about 230.
The uptrend prior to turning the filter on may have been because I was running my dehumidifier prior to the tests, and I have some small filters in that. (Yes, for some reason my house gets very humid–at times over 80%–despite the fact that Beijing is a semi-desert. I think it’s a problem with the plumbing system.)
Regardless, this data suggests that the effect of the filter was NOT a confound of calibration.