Attached garages: the highest-risk source in most homes

The single highest-risk indoor air source in most North American homes is an attached garage. Batterman et al. and many follow-up studies have shown that vehicle emissions, lawn-equipment exhaust, paint, solvents, and stored fuels leak into the adjoining house at rates that produce measurable indoor concentrations of CO, benzene, PAHs, and VOCs.

Cold-start vehicle emissions are the worst case. A vehicle's catalytic converter doesn't reach operating temperature for 60–120 seconds; during that window emissions of CO and unburned hydrocarbons can be 100× higher than steady-state. "Just letting the car warm up in the garage" is the highest-risk version of this pattern; idling at all in an attached garage is a non-trivial CO event.

Terrestream will see the VOC and PM_2.5 components of garage intrusion and flag the pattern, but it cannot detect the CO. This is exactly the case the CO disambiguation article warns about: an attached garage is one of the few residential situations where CO levels can climb fast enough to be dangerous. A UL 2034 / CSA 6.19 CO alarm in any room adjoining an attached garage is essential.

Mitigations: never idle in the garage, even briefly; back the car out quickly when starting cold; weather-seal the door between house and garage; install a passive vent or exhaust fan in the garage itself; store gasoline, paint, and solvents outside or in a detached structure if practical. Newer homes (post-2009 IRC compliance in many U.S. jurisdictions) require an air-sealed garage-to-house door, older homes often don't have one.