indoor vs outdoor

上海测试:室内比室外空气好吗?

我刚拿到第一台空气测试仪的时候,我做了一个小测试:我去了北京几个地方采样,想看看室内空气是不是比室外空气好。结果发现是

不过,那次测试有些缺点。首先,我第一台测试仪没有电池,所以我当时只能估计外面的空气污染。第二,我当时也没分析有什么因素让某些地方的空气比其它地方要好。

上海测试

现在有了很方便带电池的Dylos DC1700测试仪!可以测到室内和室外的空气。这是我最近在上海测到的空气:

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我趁去上海的机会做了新的研究:我在8月27-29号在11个地点做了14次测试,测试了室内和室外的空气。那几天上海AQI平均为158 (70微克)。我主要是在法租界采样,另外去了复旦大学。所有的采样点都不用空气净化器。

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我尽量去更多地方做测试,连公共厕所都进行测试了:

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结果

我分析了0.5微米颗粒(上面的图片左手边的数值);这些小颗粒跟政府的PM 2.5数值相关性很高。14次测试的结果是这样的,红色线是外面的污染程度:

 

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平均来说,室内的空气污染只有外面的69%。这个结果符合我在北京做的测试结果,也符合美国环保局的测试结果

为什么有的地方空气更好?

后来我想知道为什么有的地方的空气更好,有的地方更脏。比如说,我回家的时候(还没开净化器),我家里只有外面空气的30%的颗粒!但是我去的公厕有134%的颗粒–比外面还要糟糕。

我分析了最简单的原因:门窗关着还是开着?颗粒污染主要来源是外面的空气。而且,如果关门窗,颗粒会慢慢下降而落在地上。所以关门窗的时候,室内的空气应该更好。

所以我先分析了所有开着门窗的地方:

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很恐怖!如果开门窗,我呼吸的空气基本上跟外面一样脏(92%)。下面的图是公厕的实拍:

图片 6

不过,在平时关门窗的地方,结果很不一样:

图片 7

关门窗的地方只有外面的57%的颗粒。下面的图是在复旦大学的实拍:

图片 8

还有,关门窗的地方的空气总是比外面要好。只有一个例外:圆圆餐厅有(115%)。这些颗粒物应该是来自炒菜。

结论

结论很简单:我们一般在室内呼吸的空气比外面的要好,就算不用空气净化器。

建议

  1. 尽可能在室内运动。我在中国的时候在健身房锻炼身体(我在外面的时候戴口罩)。
  2. 在咖啡厅和酒吧,我一般选择坐在里面而不是外面。

最后一个例外

值得记住:有时候空气污染来自室内。一般这种情况是因为刷墙或者装修。如果家里有那种装修的味道,有时候开门窗更好(或者用活性炭)。

这意味着室内空气安全吗?

我说室内空气比外面好,但是要记住:“更好”不等于“安全”。14次测试当中,没有任何地方低于世界卫生组织的安全上限:

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Does Chinese New Year Affect Inside Air?

对不起,此内容只适用于美式英文。 For the sake of viewer convenience, the content is shown below in the alternative language. You may click the link to switch the active language.

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:

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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):

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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:

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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.

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How Much Does Outside Air Pollution Affect Indoor Air?

对不起,此内容只适用于美式英文。 For the sake of viewer convenience, the content is shown below in the alternative language. You may click the link to switch the active language.

I’ve always wondered: how much does outdoor air pollution affect the air in my home? Even if I keep my windows closed, if the air outside gets really polluted, does air in my home get a lot worse?

To answer that question, my Smart Air collaborator Anna has been taking particle counts in her Beijing apartment everyday for the last two months and comparing the counts to outside pollution. Here’s how she does it:

When she gets home from work (and before she turns on her DIY filter), she uses a particle counter to measure how much particulate matter is in her room and compares it to outside air pollution from the US Embassy Twitter feed. Her windows are always closed, and her home is near Chaoyangmen, which is in central Beijing.

Here’s the data from 41 days:

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Inside and outside pollution correlate at = .71, which is quite strong. Compare that to the = .89 correlation between air outside my home and the US Embassy numbers.

With this data, we can actually start to predict how polluted the air is inside based on how polluted it is outside. To do that, I removed a few outlying datapoints and plotted a regression line:

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So when does indoor air pollution get dangerous?

To answer that question, we first have to define “dangerous.” I use the WHO limit of concentration of 25 micrograms per meter cubed.

Then we need to convert the 0.5 micron measurements from my Dylos machine into official concentration numbers. That’s tough, but we can get a rough estimate based on my prior tests and from a formula Dylos published to convert to micrograms (0.5 microns – 2.5 microns)/100. My data shows that the WHO limit of 25 micrograms is equivalent to about 4,000 on the Dylos; according to the Dylos formula, that’s around 2,500. Using either standard, the graph above shows that the air in Anna’s home is frequently over the WHO pollution limit.

If we round those numbers up to a very conservative 5,000, we can reach a rough conclusion:

Rough rule: If outside concentration is above 40 (AQI 112), the indoor counts are very likely to be above the 5,000 limit.

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Conclusion: Outdoor air pollution strongly affects indoor air pollution, and indoor air is often more polluted than the WHO limit–even with the windows closed.

Now, remember this data is from one apartment in Beijing. The numbers probably vary a lot between apartments that have better or worse seals around the windows. The numbers probably also vary between downtown and farther out in the suburbs. That said, this data convinces me that indoor air in China frequently has dangerous levels of particulate.

As always, I’m posting the data for fellow nerds below.

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Smoking is Worse than Outdoor Air Pollution

对不起,此内容只适用于美式英文。 For the sake of viewer convenience, the content is shown below in the alternative language. You may click the link to switch the active language.

A while back, I posted data I collected from places around Beijing showing that indoor air is consistently cleaner than outdoor air. When I analyzed that data, I excluded places that allow smoking, but I’m posting the data here now:

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Even on days where outside air was bad (AQI ~ 180), the air was even worse in the cafes that allow smoking. This is even more surprising because:

  1. I was not sitting in the smoking section.
  2. The smoke was not very noticeable. (I hate smoking, but the air seemed good enough to me that I sat there and worked.)

My guess is things are much worse in smoky bars and clubs, where the smoke is so thick my clothes smell like smoke the next day.

Conclusion: Indoor air is better than outdoor air in China, but you lose any advantage once people start smoking–even if you’re in the non-smoking section.

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Does Air Conditioning Bring in Dirty Outside Air?

对不起,此内容只适用于美式英文。 For the sake of viewer convenience, the content is shown below in the alternative language. You may click the link to switch the active language.

A question that I get asked often (and that I have always wondered about) is whether my wall-mounted air conditioner is bringing in dirty air from outside. If so, it’d be safer not to use it, especially on really bad days.

My short answer is no. To explain why, I’ve got three points of evidence:

  1. How air conditioners work. Regular wall-mounted air conditioners in China do have a unit outside connected with tubes to the inside, but that tube is not bringing in outside air. It’s passing coolant, and letting heat escape outside.

So where does the air it’s blowing come from? If you look around your air conditioner, you’ll probably discover that it works like mine: it brings air from the top, runs it over the cooling coils, and blows it out the front. It’s recycling indoor air, not bringing in outdoor air.

  1. Tests of the air coming out of the air conditioner. (See a live test here.) I’ve held my particle counter up into the air coming out of my AC unit, and it’s no different from the ambient room air. I’ve also compared that air to outside air on very dirty days, and the air coming out of the AC is nowhere near as dirty as outside air.

(I did this test when I had just turned on my AC. If the AC were bringing in dirty air and I were to test the exhaust after I had been running the AC for a long time, then my whole room would be dirty, not just the exhaust.)

  1. Tests of the ambient room air before and after turning the AC on. Results? AC makes basically no difference.

Here’s what happened in one test after turning the AC on:

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And here is the average effect over 7 different tests in my bedroom.

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In each test, I ran my particle counter for 30 minutes to get a baseline. Then I turned on the AC for 30 minutes. Here I’m comparing the numbers just before I turned the AC on and 30 minutes later. As you can see, there’s basically no effect. If anything, PM 2.5 goes down slightly. This could be because of the coarse filter in the AC unit. Or it could be random variation.

Conclusion: If it’s hot outside, don’t sweat it. Use your AC.

Central AC: I should note that these tests are of wall-mounted AC units in China. Central air conditioning may work differently.

For those interested, I’m pasting the detailed data below.

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How Safe is Indoor Air?

对不起,此内容只适用于美式英文。 For the sake of viewer convenience, the content is shown below in the alternative language. You may click the link to switch the active language.

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:

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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:

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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.

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