## How Big of a Room Can the Cannon Clean?

After I tested every fan in China I could find with a flat front…

…the results of one fan stood out above the rest. Because of its appearance and how much butt it kicked, I called it “the Cannon.”

In Anna’s 15m2 Beijing bedroom, it crushed particulates. It removed 97% of the small .5 microns particles.

But how well can it do in larger rooms? Most companies estimate this using flow rate, but those calculations depend on how well houses are sealed and how dirty the air outside is. Since air outside is dirtier in China, I think it’s necessary to do real-world tests.

Methods

Fortunately, I moved into a large 4-bedroom apartment, with a 30.5m2 living room. I tested the Cannon six times on high and tracked particulates with a Dylos particle counter. Each test lasted at least three hours.

I calculated the percentage reduction in particles from the first hour to the last hour.

This test also serves as an important extension of the earlier room tests because:

1. These tests were run in the daytime. Several people have the intuition that pollution goes down at night because people are less active and fewer cars are on the road (but the data shows that intuition is false–PM 2.5 pollution is worst in the middle of the night in Beijing).
2. These tests were run while people were moving around in the room and opening the door to the outside. This is more conservative than the nighttime tests because the Cannon has to fight influxes of outdoor air.

Results

Over six tests, the Cannon removed 92% of .5 micron particles and 89% of 2.5 micron particles even with people moving around and opening doors.

In previous tests, 2.5 micron reductions were usually slightly larger than .5 micron reductions, so it’s a little surprising that the 2.5 micron reduction was 3% lower than the .5 micron reduction. My guess is that this is because people were moving in the room, and human movement affects the 2.5 micron readings much more than the .5 micron readings.

Conclusion

The Cannon can clean rooms at least 30.5 m2, which puts it above the 22.3 m2 Blue Air recommends for their 3,6000 RMB 203 model.

As always, I’m posting the raw data and more details on the methods for fellow nerds below.

## Test: Do you have to run your purifier all day?

Some purifier companies recommend running your purifier all day. But is that really necessary? If so, isn’t that a big waste of your filters and energy?

Method

To get to the bottom of it, I set a Smart Air Cannon on a timer to turn on for two hours everyday in a 13.5m2 Beijing bedroom:

I put a particle counter in the room to take measurements every minute. I did the test while I was on vacation, so there was no influence of me opening and closing doors.

Results

After six days, I came back and saw how long it took to the Cannon to clean the air each time it came on. Here’s what six days of data looked like for the small .5 micron particles:

Over the six test days, the air in Beijing became progressively worse. But on each day, it was clear when the Cannon turned on and off. The dropoffs were sharp, showing the Cannon was working quickly.

I averaged over the six test days to find out how long it took the Cannon to clean the air on average.

Conclusion

On average, the Cannon cut .5 micron particulates in half in 10 minutes. By 20 minutes, it removed 80%.

Bottom line: Powerful purifiers like the Cannon clean the air very quickly, so I see no need to run the purifier while I’m not at home.

Can I turn it off while I sleep?

The data can also answer another question a few people have asked me: “I don’t want to hear the fan while I sleep, so can I run it for an hour and then turn it off while I sleep?

In the tests, the air got dirty very quickly after the Cannon turned off (even though the windows and doors were closed). Dirty air is entering our homes constantly, even though we can’t see it.

Bottom line: I do NOT recommend turning the purifier off while you sleep.

Open Data

As always, I’m publishing the raw data and more details on the methods below. I’ll also be publishing data from similar tests in a much larger room (30.5m2) and for tests when I’m at home and moving in the room.

## Does Chinese New Year Affect Inside Air?

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.

## The Really Small Stuff

To test how effective the DIY filter is, I’ve been posting lots of data on 2.5 micron particles. But my particle counter also measures smaller 0.5 micron particles, and I’ve been looking closely at that lately.

Remember that the “PM 2.5” you hear in the news is different from the numbers on most particle counters. Laser particle counters count X particle size and above. Government machines estimate the weight (mass) of particles of X particle size and below.

0.5 micron particles are tougher to get rid of, so how well does the DIY filter do? To answer that question, my collaborator Anna tested the DIY filter we’re shipping from Smart Air on three different nights in her home. Here’s the average amount of 0.5 micron particles the DIY filter removed, with 2.5 micron particles as a comparison:

As with my most recent test, I calculated percent reduction compared to the room air before turning on the filter, rather than compared to outside air. This is a more rigorous and conservative way to calculate effectiveness than comparing reductions versus outside air.

Conclusion: On average, the DIY filter got 92% of the 2.5 micron particles and 84% of the 0.5 micron particles. Although 0.5 micron particles are smaller and harder to catch, the DIY filter is still getting 84% of them.

(Update from the future! Want more data? I do! These results are replicated in 200 glorious days of a longevity test here.)

As always, I’m detailing my methods for data nerds here:

more

Methods: For the DIY tests, we used the same HEPA and Meidi fan we’re shipping from Smart Air. As in the tests I reported earlier, my Smart Air collaborator Anna took a baseline measurement of her bedroom air with the Dylos “beast” particle counter, then she turned on the DIY filter and switched the particle counter to take hourly measurements.

Anna took the baseline measurement before she went to bed, and she set the particle counter to take hourly measurements until she woke up in the morning. To calculate effectiveness, we averaged the 0.5 micron readings for the four hours prior to waking up, and calculated the percentage reduction from the baseline reading (before the DIY filter was turned on). As I’ve said before, I think this method of calculating effectiveness is more rigorous than calculating as reduction from outside because indoor air is cleaner than outdoor air.

As always, the DIY filter and particle counter were on opposite sides of the room. The filter was against the west wall. The particle counter was close to the east wall, on top of a sofa, like this:

Anna’s room is 15 meters squared. The windows were closed at all times, and the bedroom door was mostly closed before Anna went to bed, and it was always closed during sleeping hours.

The raw data is here:

If you look at the outside AQI, you’ll see that the first test (9/9) has the most stable outside AQI counts. Thus, that number is probably the most reliable. It also happens to be the average of the three numbers.

## How Safe is Indoor Air?

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.