Sampling programs were undertaken to establish air polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) concentrations at a semirural site on the northwest coast of England in autumn and summer and to investigate factors causing their variability. Changing source inputs, meteorological parameters, air masses, and the impact of a festival when it is customary to light fireworks and bonfires were investigated. Various lines of evidence from the study point to diffuse, combustion-related sources being a major influence on ambient air concentrations. Higher PCDD/F concentrations were generally associated with air masses that had originated and moved over land, particularly during periods of low ambient temperature. Low concentrations were associated with air masses that had arrived from the Atlantic Ocean/Irish Sea to the west of the sampling site and had little or no contact with urban/industrialized areas. Concentrations in the autumn months (1 to 20 pg m-3 of ∑4-8CDD/Fs) were 2 to 10 times higher than those found in the summer (0.4 to 0.7 pg m-3 of ∑4-8CDD/Fs). Concentrations in the autumn rose as ambient temperatures decreased; the evidence that diffuse domestic heating (combustion) sources are responsible for this increase is discussed. A sample with concentrations 2 to 3 times above ambient levels was associated with a specific air mass originating from the Mediterranean that moved over Spain and France toward the United Kingdom (UK). Concentrations also increased substantially, by a factor of >10, during the autumnal festival. It is estimated that 30 g of sum of toxic equivalents (∑TEQ) of PCDD/Fs may have been emitted into the UK atmosphere during a 3-day period at this time. This amount is 5−14% of the estimated contemporary annual national primary emissions of PCDD/Fs in the UK and implicates uncontrolled burning of wood, garden refuse, and wastes on the bonfires and/or the lighting of fireworks as diffuse sources of PCDD/Fs.