How accurate are common particle counters? Comparison test

When I started Smart Air in 2013, I wanted to buy a particle counter, and I had basically two options. I could buy a US$260 Dylos or spend thousands of dollars on the crazy expensive particle counters.

Since then, the market has exploded with new particle counters as cheap as 99 RMB. But are they any good?

Putting Particle Counters to the Test

To get to the bottom of it, we tested three popular particle counters on the market. We tested the Dylos DC1700, the Origins Laser Egg, and a new particle counter called the AirVisual Node.



Dylos DC1100
Dylos DC1100


The Dylos is the trusty particle counter Smart Air has been using since 2013.


Laser Egg

Origins Laser Egg
Origins Laser Egg


The Laser Egg is the popular, more technologically savvy device.



Air Visual Node
Air Visual Node

And the Node is a fancier version, including a large screen, richer information, pollution forecasts, better user experience and even a CO2 monitor.



The Government Comparison

We placed the machines outside the Smart Air office on Dongzhimen Waidajie, about 1.3km away from the government PM2.5 monitor at the Agricultural Exhibition Center.


Map of Smart Air Office and Agricultural Exhibition Centre



We ran the machines for six days. The Laser Egg and the Node give output in PM2.5 micrograms. The Dylos gives number of 0.5 micron particles, so we converted it to PM2.5 micrograms using the semi-official formula (0.5 microns – 2.5 microns)/100.

Here are the results for the first (72-hour) test outside our office in Beijing:




Low-Concentration Tests


Next we tested on days with extraordinarily low PM2.5. That’s helpful because concentrations in homes—where most people use particle counters—are also typically low. So this data is good for testing how good the devices are at low concentration levels. We ran tests for 48 hours whilst the skies were clear.

Low concentration tests
Low concentration tests


Eyeballing both graphs, all three machines did a pretty good job of tracking the official numbers. Combining both tests, we found that both the AirVisual Node and the Laser Egg correlated r = 0.98 with the official PM2.5 numbers. For non-nerds, 0.98 is incredibly close to identical! The Dylos had the lowest correlation at r = 0.90, but still incredibly high (and similar to our previous test). These correlations are all extremely high and suggest that these particles counters are tracking government data well.


Average Deviation


Another way to measure accuracy is to look at on average how far the numbers were from the government data. The Node was the closest: it was off from the official numbers by an average of 4.8µg/m3. The Laser Egg was consistently further than the government machine, with an average deviation of 6.5µg/m3. The Dylos was off by an average of 9.1µg/m3.


Low concentration PM2.5 deviation from Agricultural Center


Perhaps one worry to note is that the Laser Egg was consistently under-estimating PM2.5 while air pollution was in the lower range. This means there could be a risk that the Laser Egg underestimates the real pollution levels in the home, giving a false sense of security. However, even these deviations were not large.


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The Airpocalypse Test

To test accuracy at extremely high concentrations, we burned a cigarette in a closed 15m3 room. Our goal here was to see how well the particle counters were at reading concentration levels over a whole range of values, including toxic levels. With the help of cigarettes and a partner NGO in Beijing, we managed to get the concentration above 1,000µg/m3!

Airpocalypse test setup

For this test we also has another machine (Sibata LD-6S) on hand as a reference. This is an industrial PM2.5 dust indicator, with an accuracy of ±10% and repeatability error of ±2%. Thus, we used the LD-6S as our baseline.


Airpocalypse test results
Airpocalypse test results


Looking at the data, it’s immediately clear that the Laser Egg and the Dylos had a hard time keeping up with these really high levels of concentration. In contrast, the Node and the LD-6S matched very closely, and were able to measure values over 1,000µg/m3. The chances of you needing to measure these values outside of experiments are very slim, but it shows that the Node is more accurate at these high levels.


Take-Home Message


Overall, the three particle counters were reasonably accurate compared to the government machines. In the estimation of the Smart Air team, all of them are suitable for giving an approximate AQI value in your home. Of all three, the Node scored the highest, with the lowest deviation from the government machines in both outdoor tests and the highest accuracy in the “crazy bad” test.




Since all three machines are reasonably accurate, the question then really comes down to: How easy it is to use the device? And what features do they have?


The Dylos (1800 RMB)

The Dylos easily loses this fight. It has no phone connectivity, and downloading the data is a terrible pain—and that’s if you have one of the old school pin connecter cables.


The Laser Egg (499RMB)

The Laser Egg is an entry-point particle counter. It gives reasonably accurate results with a simple interface. It’s not feature rich, but it does what it says on the box.


The AirVisual Node (988RMB)

 To our eyes, the Node offers the best features. For starters, it can measure CO2, temperature, and humidity. That makes it more of an ‘environment monitor’ than just a particle monitor. CO2 is important if you have lots of people in a small space as it can give an indication of how confined the space is. If you have indoor sources of air pollution (VOCs) like new furniture or remodelling, high CO2 levels can mean that those indoor pollutants are building up. Its user design shows AQI and CO2 for the past 24 hours both indoors and outdoors, a forecast for the coming days, and suggestions to help you decide when to open your windows and wear a mask. We’ve found these features helpful in our office.


After passing our tests, we will start shipping the AirVisual Node through our Taobao shop  and website. It’s a great option for anybody wanting a solid device for both home use and research (if you’re a nerd like us). Go take a look!

Over the next few months, we hope to get a larger pool of particle counters together and run more extensive tests. This is only the beginning! Once we’ve independently verified more devices, we may well be adding them to our shop as well.



Winter (Air) is Coming

Another year, another Beijing winter. As winter arrives to Beijing, I got to wondering: how much worse is winter air?

To get to the bottom of it, I analyzed the last seven years of US Embassy PM 2.5 data. I found that the capital’s air has averaged 111 micrograms in the winter versus 92 micrograms for the rest of the year.


Just how bad is that? The WHO 24-hour PM 2.5 limit is 25 micrograms. That means Beijing’s summers average three times the limit, and winters average over four times limit.


Breathe safe this winter!



What time of day is PM 2.5 the lowest?

My intuition has always been that air pollution is lowest at night because there are fewer cars on the road and fewer factories humming. Apparently I’m not alone: 139 voted for their guess about what time of day has the lowest PM 2.5, and night time came in first:


I also know people who arrange their schedules to work out in the morning to avoid the worst pollution. But how accurate are our intuitions?

To get to the bottom of it, I analyzed thousands of hours of PM 2.5 data from the US Embassy in Beijing. When I finally got the answer, I was surprised:


Instead of during the quiet of night, it’s the afternoon–right around rush hour–that PM 2.5 is the lowest. So if you’re planning a picnic or insist on exercising outside, you’re usually best off between noon and 6pm.

What about other cities?

Is that how PM 2.5 generally works, or is it unique to Beijing’s activity or climate? Fortunately, US consulates in several other cities publish their historical data.


In terms of climate, Shenyang is pretty similar to Beijing, and its daily PM 2.5 patterns are very similar (although night time isn’t quite as bad as Beijing):



The pattern in Chengdu is similar. PM 2.5 is lowest in the afternoon and highest in the morning.



The picture starts changing when you get to the south. In Shanghai, PM 2.5 is lowest in the afternoon (like the north), but there’s also a dip in the very early morning:



Guangzhou’s pattern was virtually identical to Shanghai’s: a dip in the very early morning and a dip in the afternoon.


Bottom Line: When is it safest to be outside?

In all five cities, the afternoon had the lowest PM 2.5 levels. And in contrast to many people’s intuition, the night time had the worst air in several cities. Thus, you’re usually best off organizing your picnic or tai chi in the afternoon.

But keep in mind that the air is NEVER safe on average in any of the five cities at any time of day. So take “safest” with a grain of salt!


Open Data

All of the data is available from the US embassy and consulate websites. Thanks to Josh Malina for collecting and analyzing the consulate data.


Smart Air WeChat

Anyone who knows me knows that I’m a Luddite. I bought my FIRST smartphone last August.

So we should be all the more surprised that I just created an official Smart Air WeChat account! Add me, and I’ll keep you up to date on all of our workshops, new tests, and projects.

Use WeChat to scan this, and you can add the official Smart Air WeChat account! I’ll post updates about workshops, new machines, and new data.  ID: SmartAir聪明空气


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How long do Smart Air HEPAs last? At least 90 days

The most frequent question people ask me these days is: how long does the HEPA last?

This question is important because replacement HEPAs are the biggest long-term cost of clean air. IQ Air charges $370 for its filters. So if you have to replace the HEPA every two weeks, the DIY might not save you money in the long run.

What’s Wrong with the Standard Numbers? I’ve been slow to answer because I wanted good data. Lots of filter companies give a single number (often 6 months), but they don’t say whether those numbers are for European air or Chinese air. If they last 6 months in Swedish air, I highly doubt they’d last 6 months in China’s thick air.

Another problem is that “6 months” doesn’t tell you how much you can use it per day. Is that 6 months for 24 hours a day? 8 hours a day?

A More Empirical Answer: So to get a concrete answer to this question, I think the best way is to get a new HEPA and test how effective it is every day. Over time, we can see when it starts to lose its effectiveness.

It’s been tough to start that test because I’ve been busy running tests of new HEPAs, fans, and other purifiers. But thankfully my collaborator Gus volunteered his bedroom as a laboratory, and we now have data from 90 days of tests.

Limitation: However, before I reveal the results, I discovered a flaw in the study. I was looking at Gus’s data, and I noticed the particle counts in his room before he turned on the purifier were a lot lower than they were in my room. Why?

Then we realized that Gus’s roommate was running his large Blue Air 24 hours a day. That made the air in his house cleaner than it would be in a normal house. Fortunately for science, the roommate moved his Blue Air out on Day 57, so we now have a normal testing environment.

However, this created two limitations in the data:

  1. It may inflate our estimates of the longevity of the filter because the air was cleaner than it would be in a normal Beijing apartment.
  2. It artificially lowered the estimates of effectiveness for the first 60 days. (At the end of the post, I compare data from when the Blue Air was being used nearby and after it was gone.) This also makes it hard to tell whether there were any slight changes in effectiveness over time.

Gus will start completely new tests after we finish these tests, so we’ll have a better answer later. We debated whether publishing this data would be useful. In the end, I decided that it’s better to give people some idea of the longevity, as long as I make it clear that there are limitations with the data.

Method: Gus used the Original DIY and the same HEPAs we ship from Smart Air every night in the 12.3 m2 bedroom in his Beijing apartment.

Gus used the same method as my previous tests to calculate effectiveness–the percentage reduction of particulates from the room air overnight.

To smooth out the variability in any single datapoint, I averaged the effectiveness over each 10-day period.

Here’s an example of what one of the 90 days looks like:


Here’s what the HEPA looks like after 90 days:


Results: After 90 days, the HEPA is still getting as many .5 micron particles as in my earlier tests:


Conclusion: This suggests Smart Air HEPAs last at least 90 days without losing effectiveness. Stay tuned to see how the next 90 days turn out!

Remember: these results can only be generalized to Smart Air HEPAs. HEPAs from different manufacturers vary in thickness and quality.

Number of hours: At the end of 90 days, Gus has used the HEPA for a total of 660 hours, an average of 7.33 hours per day. When we get a

total estimate of the number of hours the HEPA lasts, you can plug in the number of hours you use the filter per day to get an exact number of days for your own use.

For fellow data nerds, I’m posting more details about the tests below.


The Effect of Having a Second Purifier on Nearby: As I described in the main text, Gus’s roommate had a large Blue Air running for the first 56 testing days. My hypothesis was that the Blue Air was artificially lowering the baseline particle counts, which makes it harder to get high-percentage reductions.

We were able to test this hypothesis after his roommate prepared to move to Hong Kong and shipped his Blue Air there ahead of time.

To test my hypothesis, I compared the effectiveness (1) during the 56 days that the Blue Air was running and (2) the 34 days after his roommate stopped using the Blue Air:


The estimates of effectiveness rose to 86%–even higher than my original tests. This suggests that the earlier numbers were artificially low.

It also means that we can’t interpret any changes before Day 70:


The numbers go up when Gus’s roommate stopped using his purifier, but I think it’s wrong to say the DIY became more effective.

What we can say is that, even after 90 days of usage, the HEPA is still working as well as in my original tests.

Can we generalize the results? These tests are great for people living in China because the tests are based on Beijing air. However, the filter may last longer in places with cleaner air (for example, Hong Kong) and shorter in places with dirtier air (for example, Taiyuan).

In addition, fluctuations in AQI could make these estimates longer or shorter. For example, winter air tends to be more polluted.

Dropoff After 10 Days? The dot for the first 10 days is slightly higher than the later days, which suggests a dropoff after 10 days. However, I think that is not the case. In the first 10 days, Gus was still learning how to use the particle counter, and he failed to set it properly or get a good baseline reading on four of the days, so we couldn’t calculate effectiveness. That means the estimate for the first 10 days is less reliable.

What’s Going on at Day 30? The third datapoint (days 21-30) is the lowest. That is probably because the outdoor air was particularly bad in Beijing during that time. The numbers rebound slightly during the next 20 days.

Why HEPAs get better with age (to a point). Many people have the intuition that filters get worse over time, but HEPA filters actually get better at removing particles. That’s because the more you use it, the more particles fill up the pores in the HEPA, and it becomes harder for particles to get through.

The problem is that it also becomes harder for clean air to get through. So eventually, the HEPA will get so full of particles that it won’t let enough air through. That’s how the HEPA eventually dies.

Raw Data: The file is too large to display all of the raw data here, so I’m making it downloadable from my University of Virginia homepage (click here to download).