More than 80% of secondhand smoke (SHS) is invisible to the human eye, and smokers are often unaware of how much SHS is produced when they smoke at home. Very little is known about how long SHS particles remain suspended in the air within household settings.


Data from a series of studies where fine Particulate Matter (PM 2.5 ) concentrations were measured every minute in homes were used to identify the PM 2.5 peak produced by SHS from the last cigarette smoked of the day. The time taken for this peak to reduce by 50% was calculated, as was the time between the peak and a return to the World Health Organization’s (WHO) 24-hr guidance value for PM 2.5 of 25 µg/m 3 .


From more than 230 days worth of 1-min resolved PM 2.5 data gathered from 103 smoking households, 140 suitable peaks were identified. The median (interquartile range [IQR]) for the time for the SHS peak to decay by 50% was 55 (23–116) min, and the median (IQR) for the peak to reduce to the WHO guidance value for PM 2.5 was 160 (90–313) min.


SHS remains in household air for a considerable period after smoking a cigarette. This information is likely to be of use in public health information campaigns and in interventions to encourage smokers to make their home smoke-free.

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