To determine if a low-cost particle counter, the Dylos DC 1700, can be used in homes to quantify second-hand smoke (SHS) concentrations. Participants were recruited from a hospital-based study of attitudes on smoking. Two photometric devices (Dylos DC1700 and Sidepak AM510 Personal Aerosol Monitor) capable of measuring and logging concentrations of particulate matter were placed in smoking and non-smoking homes for approximately 24h. Acquired data were randomly allocated to one of two groups: one was used to generate a calibration equation using regression techniques; the second was used for validation of the generated model. The mean difference and limits of agreement between the two instruments were calculated using the validation dataset. Summary air-quality results were also compared across the entire dataset. Over 500 001 minute concentration measurements were collected from 34 homes. 25 301 randomly selected paired-measurements were used to generate a calibration equation (R(2) 0.86) converting the particle number concentration from the Dylos to a mass concentration of PM2.5 as measured by the Sidepak. The mean difference (limits of agreement) between the remaining 25 102 paired measurements was -0.09 (-49.7 to 49.5) with 3.2% of values outside the limits of agreement. Differences in the air quality information generated by the two instruments were generally small and unlikely to impact on user interpretation. The Dylos appears to be a valid instrument for measuring PM2.5 in household settings. The Dylos may be useful in air quality-based interventions designed to change smokers' behaviours with the possibility of encouraging cessation and/or smoke-free homes. Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://group.bmj.com/group/rights-licensing/permissions.