I have been observing a strange air quality pattern on my indoor Touch device. Sometime between 9 and midnight every night PM2.5 readings shoot up about 10x. They stay high throughout the night and then trend down from about 6am to mid-afternoon. At first I thought it might be outside air infiltration due to the cold but often the indoor readings are higher than the outdoor. My furnace is old and I thought it might be the culprit but putting the sensor on top of the vent showed that the hot air is cleaner than other indoor air. The pattern also occurs on nights we don’t run the dishwasher. Any other ideas on what could be causing this?
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Humidifier turning on and using non-distilled water?
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This turned out to be it! Turned off the ultrasonic humidifier and the problem went away. Humidifiers did not turn up in the standard lists I found of indoor air pollution. This really needs to be better known. Thanks for the help.
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It’s more of a quirk of how sensors of the type PurpleAir use react to humidity. Moving the sensor further away from your humidifier would blunt the effect. It’s why there’s a large adjustment in the calculations to compensate for humidity.
That said, any sort of non-evaporative humidifier will eject whatever dissolved solids are in the water into the air along with the water. This is why there are frequent complaints about ‘white dust’ deposits in areas where these types of humidifiers are used.
I don’t know offhand if there are any studies of the health effects of breathing untreated vaporised water.