As I write, the air quality in most of Northern California is pretty good but it was horrible about a week ago and could get bad again during the rest of the fire season.  A few weeks ago I wrote about various government and privately-owned air monitors in our communities, including

Larry Magid

ones from the Purple Air, that can be installed in people’s yards or inside their homes. I’m fortunate enough to live near a couple of those sensors that my neighbors have purchased and installed, but many people live miles from the nearest sensor, so the data they get may not reflect their immediate environment.

To find out what’s involved in installing one’s own monitor, I asked Purple Air to send me a pair of them. One, the $199 Purple One PA-1 Indoor, is inside my house, in my home office. The other, the $249 PA-II, is mounted outside my house.

As you’d expect, these two sensors are giving off different readings. Most of the time, the air quality inside my house is better than outside, but not always. As I’ve learned over the past few days, there are many potential sources of indoor pollution including dust and even the fine particles emitted from laser printers and off-gasses from equipment, including computers. And even if you have all your windows and doors closed, most houses are somewhat permeable so outside air — and pollution — can get in.


You start by configuring the Purple Air sensors to your Wifi network so they can report their data to the company to be displayed on a map either publicly or privately. I made both of my sensors public so that my neighbors can know the air quality in our neighborhood as well as the air inside at least one room in my house.

The indoor sensor can be placed on any table or counter. The outdoor sensor is designed to be mounted on an outside wall. Both the indoor and outdoor sensors need to be placed near an outlet and have access to WiFi. The company also makes a more expensive version with an SD card that can capture data if there is no WiFi. The outdoor sensors can also be used indoors, but they must be mounted to a wall.

The indoor sensor has a color-coded light on top that tells you from across the room if the air is good (green), moderate (yellow), or bad. While I appreciate that light during waking hours, I wish there were a way to turn it off if you have it in your bedroom when sleeping.

How it works and what it measures

As Purple Air explains in a FAQ, “sensors measure airborne particulate matter (PM). Particulate matter describes solid particles suspended in air; this includes dust, smoke, and other organic and inorganic particles.”  Like most consumer-level air monitoring systems, Purple Air uses laser counters to measure particulate matter. A fan draws air past the laser beam so that particles in the air will reflect light to a detection plate.  The FAQ goes on to say that the reflection is measured as a pulse and the length of the pulse determines the size of the particle, while the number of pulses determines the particle count.

The PM scale includes PM10 (coarse), which are 10 microns or less, and according to the California Air Resources Board, are inhalable into the lungs and can induce adverse health effects. Fine particulate matter is defined as particles that are 2.5 microns or less in diameter (PM2.5). The Air Resources Board says that PM10 and PM2.5 often derive from different emissions sources and also have different chemical compositions. Emissions from the combustion of gasoline, oil, diesel fuel, or wood produce much of the PM2.5 pollution found in outdoor air, as well as a significant proportion of PM10. PM10 also includes dust from construction sites, landfills and agriculture, wildfires and brush/waste burning, industrial sources, wind-blown dust from open lands, pollen and fragments of bacteria. The Purple Air sensor also detects PM1 (microfine), which is even smaller than PM2.5.

Whether you have your own sensor or not, you can access the public ones from the map at If you want the report to be consistent with data from the EPA’s official monitors, go to the box labeled “Map Data,” look for the word “none” and change it to LRAPA. As you move about the map, notice that the URL (web address) changes. If you save that site to your PC or mobile device, you can get back to it at any time or even share it with your neighbors.

How they differ from federally mandated sensors

The sensors used for EPA regulated measurements work by drawing air through a filter and weighing the filter. These devices and their maintenance costs are very high, making them impractical for homeowners and even some communities. As a result, there aren’t nearly as many of the federal reference sensors as we need. For example, the one nearest my house is at least 10 miles away, and the air can be considerably different depending upon local conditions and wind. And of course, they can’t measure air inside your house. The less expensive laser sensors aren’t as precise, but they do the job and seem to correlate at least roughly with the official sensors.

Air filters

Molekule Air Pro
Molekule Air Pro

There are many air filters on the market, starting at about $50 and going way up from there. By coincidence, the day after I installed the Purple Air monitor, Molekule sent me its newest air filter to review. The Molekule Air Pro costs $999 (after a $200 discount) so it’s clearly not within everyone’s price range, but according to the company, it has three times the airflow of its consumer products and can detect and filter out PM10 (coarse particles), PM2.5 (fine) and PM1 (microfine). The company says that the device has “virus, bacteria, chemical, mold and other pollutant destruction capability.”

A display on the device and on the accompanying app reports the air quality in the room where the device is located broken down by course, fine and microfine particles. The app can also be used to control the device, including setting it into Auto-Protect mode where the fan speed is determined by the room’s air quality.

I asked Molekule CEO and co-founder Jaya Rao how her product differs from less expensive air filters, and she said that the device’s PECO technology “is capable of not just filtering the air but also destroying at the microscopic level the pollutants in the air including mold and bacteria.”

Without sophisticated lab equipment, I can’t verify the company’s claims, but I can say that before I turned on the Molekule filter, the Purple Air monitor in my office was reading “moderate” air quality and, within about an hour, it switched to “good.” I can also say that the device is beautifully designed — it looks as if it could have come from Apple — with an excellent “out of box experience.” It even has a handle to make it easier to move from room to room for people who may want to move the unit into whatever room they’re in.

Dyson also makes high-end but less expensive purifiers starting at about $350. They don’t claim to “destroy” pollutants, but they do have a high-quality HEPA filter that “captures allergens, pollutants and gases.” I have two of them which also serve as cooling fans and heaters, and although its reports aren’t as detailed as those from Molekule, some models do monitor and report the air quality in the room and can adjust its speed accordingly.

Larry Magid is a tech journalist and internet safety activist.







By Kelley Wheeler

Kelley Wheeler is a Metro reporter covering political issues and general assignments. A second-generation journalist, worked with all major news outlet, she holds a vast expeirience. Kelley is a graduate of USC with degrees in journalism and English literature. She is a recipient of Yale’s Poynter Fellowship in Journalism.

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