Ohad used a portable Dylos DC 1700 laser particle counter. The 1700 has two channels: (1) particles of 0.5 microns and above and (2) 2.5 microns and above. To get a sense for how accurate this machine is, check out my comparisons of the Dylos and the official readings from the US Embassy in Beijing. Ohad also recorded outdoor PM 2.5 (ug/m3) from the nearest official measuring station. He recorded humidity from Apple Weather. Ohad was also interested in CO2 levels, so he bought a portable CO2 meter.
Air Conditions
All tests were carried out by Ohad in Israel. Israel is not famous for its air pollution, but there were plenty of hazardous days for testing. As measured by the Dylos, 0.5 micron readings outdoors ranged from 2,464 to 31,545. That rivals Beijing air! The municipal PM 2.5 readings ranged from 10.7 to 294.1 ug/m3.
Test Time
Tests were targeted for five minutes long. I excluded from analysis any test that didn't last five minutes. Ohad started measuring when he got into the car, and the Dylos continued taking measurements while he was driving. The act of opening the car door brings in outdoor air (and outdoor air usually has more particulate than enclosed spaces). Plus, human movement should kick up dust, making readings higher at the beginning. This is probably why the tests with the AC off show small declines.
Car Setup
Ohad drove his 2010 Mazda 3. Like me and like most car drivers, Ohad didn't know what filter was in his car. As he put it:
"I have no information about the filters I have in my car. I assume they are original filters from Mazda - but otherwise I don't even know where they are installed."
Thus, I assume these are standard car filters and not some super special filter. He did not replace the filter for the test, so the filter was not new.
For one test, Ohad drove a 2012 Mazda 6. The results were similar to the other tests.
Fan Speed
Ohad ran 8 complete tests with the fan on high, 5 complete tests with the AC completely off, and 2 tests with the AC on low. The air system was set to "recirculate" mode. I suspect this reduces particulate faster than the mode that brings in outdoor air.
Raw Data
The raw data in Excel file is available as a download here.
High Setting vs. Low Setting
There were three tests--one with AC on high, one on low, and one off--that all started around the same baseline of 30,000 0.5 micron particles, so I plotted them against each other. The results are pretty much what you'd expect:
CO2
As a fellow nerd, Ohad was also interested in measuring CO2 during three car trips. After 5 minutes, CO2 ranged from to 1,270-1,332 PPM. That's about what I've found in a closed bedroom during sleep. Those levels are well in the safe range, although I know of one study that suggests levels of 1,000 parts per million (PPM) can lower people's performance on cognitive tests. I'd like to see more research before I start worrying about 1,000 PPM. (I discuss more about safe levels of CO2 here.)