In urban and suburban settings, indoor ozone exposures can represent a sign
ificant fraction of an individual's total exposure. The decay rate, one of
the factors determining indoor ozone concentrations, is inadequately unders
tood in residences. Decay rates were calculated by introducing outdoor air
containing 80-160 parts per billion ozone into 43 residences and monitoring
the reduction in indoor concentration as a function of time. The mean deca
y rate measured in the living rooms of 43 Southern California homes was 2.8
0 +/- 1.30 hr(1), with an average ozone deposition velocity of 0.049 +/- 0.
017 cm/sec. The experimental protocol was evaluated for precision by repeat
ing measurements in one residence on five different days, collecting 44 sam
e-day replicate measurements, and by simultaneous measurements at two locat
ions in six homes. Measured decay rates were significantly correlated with
house type and the number of bedrooms. The observed decay rates were higher
in multiple-family homes and homes with fewer than three bedrooms. Homes w
ith high er surface-area-to-volume ratios had higher decay rates. The ratio
of indoor-to-outdoor ozone concentrations in homes not using air condition
ing and open windows was 68 +/- 18%, while the ratio of indoor-to-outdoor o
zone was less than 10% for the homes with air conditioning in use.