There are moves to ban smoking in outdoor areas of pubs, restaurants and cafes. Some argue that this is unnecessary as exposure to second hand smoke (SHS) is minimal. The aim of this study was to determine potential exposure of patrons to SHS in outdoor areas of eating and drinking venues.Concentrations of fine particulate matter (PM2.5) were measured in the alfresco areas of 28 cafes and pubs. Data were collected on the number of smokers present during sampling and factors that could influence PM2.5concentrations. PM2.5concentrations for periods with and without smokers were compared using paired and independent sample tests.PM2.5 concentrations were significantly increased when there was at least one smoker compared to periods with no smoking (14.25 microg/m3 and 3.98 g/m3, respectively). There was evidence of a dose response increase with mean concentrations for none, one and two or more smokers of 3.98, 10.59and 17.00microg/m3, respectively. The differences remained significant after controlling for other factors. When two or more people were smoking, average PM2.5reached levels the US Environmental Protection Agency warns may put particularly sensitive people at risk of respiratory symptoms.Smoking increases PM2.5concentrations in outdoor areas to levels that are potentially hazardous to health.