A method of preventing infiltration of pollutants into a building, wherein difference in air pressure on respective sides of a building envelope is controlled in order to prevent infiltration of pollutants, to this end obtaining a measurement of a current difference in air pressure on respective sides of the building envelope; characterized in further obtaining an estimate of the building's airtightness expressed as a correspondence between air flow (17, 18) through a building envelope and a difference in air pressure on respective sides of the building envelope; obtaining a measurement of a current concentration of pollutants in the air outside the building envelope; calculating an estimated current infiltration of pollutants per unit time through the building envelope from the building's airtightness, the current difference in air pressure on respective sides of the building envelope, and the measurement of a current concentration of pollutants in the air outside the building envelope; and if the calculated infiltration of pollutants per unit time through the building envelope is above a predetermined threshold for infiltration of pollutants per unit time and if the air pressure on the inside of the building envelope is not at least a predetermined threshold for air pressure difference higher than the air pressure on the outside of the building envelope, controlling a ventilation system in the building to increase the inside air pressure until the air pressure inside the building envelope becomes higher than the air pressure outside the building envelope with at least the predetermined threshold for air pressure difference, thus preventing or reducing infiltration of polluted air or other unwanted gases through the building envelope. to this end obtaining a measurement of a current difference in air pressure on respective sides of the building envelope; characterized in further obtaining an estimate of the building's airtightness expressed as a correspondence between air flow (17, 18) through a building envelope and a difference in air pressure on respective sides of the building envelope; obtaining a measurement of a current concentration of pollutants in the air outside the building envelope; calculating an estimated current infiltration of pollutants per unit time through the building envelope from the building's airtightness, the current difference in air pressure on respective sides of the building envelope, and the measurement of a current concentration of pollutants in the air outside the building envelope; and if the calculated infiltration of pollutants per unit time through the building envelope is above a predetermined threshold for infiltration of pollutants per unit time and if the air pressure on the inside of the building envelope is not at least a predetermined threshold for air pressure difference higher than the air pressure on the outside of the building envelope, controlling a ventilation system in the building to increase the inside air pressure until the air pressure inside the building envelope becomes higher than the air pressure outside the building envelope with at least the predetermined threshold for air pressure difference, thus preventing or reducing infiltration of polluted air or other unwanted gases through the building envelope.