Your commute is part of your exposure, and Terrestream cannot measure it

Commute exposure is well characterized, and it depends heavily on how you travel. A driver in stop-and-go traffic breathes cabin air that picks up NO₂, ultrafine particles, and other exhaust from the vehicles a few meters ahead. A California Air Resources Board study of Sacramento commuters found that electric light rail had the lowest particle and soot exposure, older diesel trains the highest, and cars, buses, and bikes fell in between. Running a car's air conditioning on recirculate cut exposure to fine particles, ultrafine particles, and soot by up to 75%, and cyclists cut theirs by 15 to 75% on dedicated paths away from traffic. Cyclists also breathe harder, so the same air delivers a bigger dose.

A fixed indoor sensor cannot read any of this, by physics. The Terrestream module lives at one address; the SEN66 measures the air at the sensor's location and nowhere else. Sending the device on a commute is also not a workaround: the PM intake fan and the gas sensors are not weather-sealed, the module is calibrated for indoor conditions, and the wifi backhaul disappears the moment you leave the house. The honest answer is that personal-exposure measurement is a different instrument class. WHO outdoor-air guidance exists precisely because outdoor exposure has its own monitoring infrastructure.

What does work for commute exposure: portable personal-exposure monitors (Atmotube Pro, AirVisual portable units, Plume Labs Flow) which are battery-powered, weather-tolerant, and Bluetooth-paired to a phone. Phone-app-only approximations work too: pull the nearest EPA AirNow or NAAQS reference station reading and multiply by the time you spend outdoors or in traffic. Treat the result as a floor rather than an estimate: EPA notes that people near busy roads and other pollution sources can be exposed to higher levels of air contaminants, and a regional station cannot tell you how much higher your stop-and-go commute runs. The arithmetic is rough, but it is enough to inform decisions like "is biking on this corridor a good idea this week" or "should I avoid running outdoors during the afternoon rush."

What Terrestream does help with: characterizing the indoor environment you spend the rest of your day in. For most people that is 16 hours at home and 8 hours at work, which is 80% of waking exposure once you subtract sleep. Optimizing the part of exposure the device can measure is rational even if the commute remains a blind spot. The dashboard is also useful as a comparison anchor: when the indoor PM_2.5 reading is 4 µg/m³ and the AirNow station reads 18 µg/m³, that gives you a sense of the gradient you walk through every morning. See outdoor air feeds for how the dashboard pulls outdoor reference data, dose and time-weighted exposure for the framework that ties commute and indoor exposures together, and accuracy and confidence for what the indoor reading does and does not generalize to.

This is general guidance, not a substitute for professional assessment of your specific home. Major interventions (HVAC redesign, sealing a leaky envelope, mold remediation, electrical work for fans or venting) should be done with a certified professional. For chronic problems that don't respond to the steps here, see when to call a pro.