Summary: By plotting low‐volume aerosol counts as indices of pollution in Hamilton a pattern of dual pollution cells of equal intensity centring on the downtown business area and the heavy industrial zone emerges. Pollution levels are twice as great under east winds with accompanying atmospheric stability than for winds from all other sectors. With winds from the easterly sector the industrial pollutants are forced and locked into the lower city. When a major source of industrial pollution disappears, as happened during the 1969 shutdown of the Steel Company of Canada plant, the industrial cell vanishes and the pollution count under east winds drops to a level normal for other wind directions. With a wind change from the east to the west new pollution patterns are formed within six hours. The average pollution count for the city drops substantially but in the east end it increases as the pollution haze formerly trapped in the city is released and carried eastward.