I just added two nearby outdoor sensors to my HomeKit app and have been checking them periodically in comparison to indoor sensors I just purchased to get an idea of trends. One of the sensors is right next to the beach (Venice, CA) and the other is more inland (but not THAT much more!). The beach sensor has been reading high VOC density every day, though I don’t know what level is dangerous, the other Purple Air sensor and my indoor sensors are usually zero but definitely less than 1 (microgram/meter^3). The beach is currently at 61, other sensor at 0. Is high VOC density outdoors normal?? What would cause such a large difference with a separation of only a couple miles?
Hi Catherine,
Air pollution can be very localized, all dependent on how the pollutant is introduced to the air and how air moves through the area. The same applies to both indoor and outdoor air.
The VOC sensor in PurpleAir devices is still experimental as the sensor’s sensitivity to various VOC pollutants is not well known. We don’t know what constitutes an “unsafe” or “dangerous” level. I would not use the PA VOC sensor alone to make any health decisions. If you are concerned about the readings I would contact your local air quality agency and inquire if there are any known incidents that are affecting air quality in your area.
Looking at the graphs for indoor sensors around LA, they seem to react when the area is occupied. Different sensors also seem to have different baselines – I see several that bottom out around 50 and others at 25. Without knowing more about the spaces they’re sensing and their use, I can’t tell if that’s significant or not.
Looking at outdoor sensors in Venice, there is a VOC sensor just west of the marina. If we think about VOC sources for that area, certainly the marina (with gas-burning boats and refueling stations) seems like a good possibility. LAX is not too far to the south and could certainly contribute pollutants as well.
Zooming out a bit, the VOC level seems fairly consistent through the area (as of right now, around 8:30am PST on 1/28), so it may be a normal reading for outside air in that part of LA. I am seeing a spike in the VOC charts that started around 8pm on 1/27. That loosely correlates with a wind shift recorded at LAX around that time.
Finally, if your sensor was newly installed, it takes time for the VOC sensor to equalize with the environment and during that time it returns no data.
Hope this helps add some perspective – please ask if you need any clarification.