I continue to be very excited about my new M2000 meter, and I brought it with me today to measure air quality during my commute and workday. Here's what I found:

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Time	Activity	CO2 (ppm)	pm2.5 ug/m3
8:00	Left house	426	2.9
8:15	Davis station	468	69.9
8:24	Red line, new car	802	29.3
8:36	Kendall station, outside	477	11
8:40	Hallways, elevators, breakfast in cafe	563	2.8
9:18	Office, door open	475	1
10:00	Closed door	514	1
10:31	Open door	553	1
11:02	Team meeting, 14 people, ~15x30	1052	1
12:04	Hallways, elevator	686	1.6
12:10	Lunch in cafe	943	2.6
12:50	Hallways, elevator	763	2.1
12:57	Office, door open	537	1
2:02	Closed door	539	1
2:31	Two people	599	1
2:54	One person, open door	587	1
3:01	Closed door, two people	598	1
3:31	Open door, zero people	538	1
4:04	Closed door, two people	591	1
4:38	Kendall station	487	36.6
4:44	Red line, old car	953	21.6
4:56	Davis station	537	134.5
4:58	Outside	445	2.25

I spent most of my workday in my office, with the door open. I closed the door for meetings, either virtual or in person. The "zero people" at 3:31 is me going on a walking 1:1: with a report and leaving my meter behind at my desk.

Currently I wear a mask on the subway (and stations) and at work when I'm not in my office or a conference room with a small number of people. Based on these readings, it would be safe from a covid perspective to remove my mask in the subway station, but given the high level of particulate pollution I might as well leave it on.

(At this point I'm not wearing a mask because I think it would be directly harmful to myself or my family to get covid, but instead because the steps we would need to take to reduce the risk of infecting others once we knew we were infected would be very inconvenient.)