Particulate matter (PM) is a leading contributor to indoor air-related health harm. While population-level estimates of harm
are typically based on outdoor PM concentrations linked to epidemiological studies, most exposure occurs indoors, particularly
in homes where people spend the majority of their time. Indoor concentrations differ from outdoor values due to the presence
of both indoor and outdoor sources, and are shaped by building characteristics, ventilation regimes, filtration, and occupant
activities. The indoor-to-outdoor (I/O) concentration ratio provides a useful metric for quantifying these differences and varies
with transport, removal, and emission processes. This paper reviews the evidence on I/O ratios in homes, focusing on fine and
coarse particles, which is currently thought to account for the majority of harm from indoor air contaminants. It evaluates how
infiltration, penetration, deposition, filtration, and indoor sources influence I/O ratios, and examines how these variations
affect estimates of population harm. The results show that relying on outdoor PM concentrations as a proxy for exposure
introduces bias. In homes with few indoor sources, effective filtration, or limited infiltration, harm is overestimated; in homes
with dominant indoor sources, harm is underestimated. We conclude that robust public health assessments should incorporate
indoor exposure dynamics and I/O ratio variability to improve exposure estimation and support the evaluation of mitigation
strategies, including air cleaning, envelope improvements, and source control.