This study quantified the contributions of both indoor- and outdoor-generated particles to the air inside hospitals and examined whether air conditioning type, working area, working hours, and ambient pollution affect these contributions. Indoor and outdoor fine and coarse particles were measured at 33 hospitals, and the building characteristics were recorded. The infiltration factor (Finf) was calculated, and the contributions of both indoor and outdoor particles to indoor air were assessed. Additionally, their influencing factors on the indoor air were evaluated. The Finf values of fine and coarse particles were higher in hospitals with window and signal split type air conditioning than in hospitals with other types of air conditioning. No significant differences in the Finf values between working areas were observed. Outdoor-generated fine and coarse particles were major contributors to the indoor air, regardless of air conditioning type and working area. Higher contributions from indoor-generated fine and coarse particles to the indoor air were recorded in clinic waiting areas and lobbies during working hours than nonworking hours. Ambient air pollutant emissions and air conditioning characteristics influenced the contributions of indoor- and outdoor-generated particles to indoor air according to a regression model. In summary, the contribution of outdoor-generated particles to indoor air must be reduced to improve occupants’ health in hospitals.