The particle concentrations outside and inside two historical churches were monitored for at least ten months. The highest levels of outdoor concentrations were recorded in winter. This was caused by high levels of particle emissions from the burning of predominantly solid fuel for domestic heating in premises around the two churches monitored. These high levels of particle concentrations declined over the warmer periods of the year with the lowest concentrations occurring in the summer months. Owing to the marked winter–summer pattern for outdoor concentrations, the particles of outdoor origin accounted for 80%–90% of the overall indoor particle concentrations in the period of predominantly cold weather conditions (December to March) and for 50%–60% in the warm period (June to September). Reducing air exchange between the external space and the church interiors by keeping windows and doors closed had a limited effect on the reduction of average particle concentrations indoors (by less than 10%). A controlled air exchange system, which would increase the ventilation of a church when the particle concentration outdoors is lower than indoors and reduce ventilation when the outdoor air is polluted, would produce a further reduction of 10% in the indoor average particle concentration. The general conclusion is that the protection of the interiors of historical churches against soiling is primarily achieved by the improved particle filtering capacity of building envelopes and the gradual reduction of the overall outdoor particle concentration. Use of air cleaning systems with particle filtration may be a viable long-term option.