When meteorologists measure relative humidity, do they take into consideration particles of pollution in the air?

Dear Tom,
When meteorologists measure relative humidity, do they take into consideration particles of pollution in the air?
— Bruce Ameismer, Chicago
Dear Bruce,
Air pollution is not taken into consideration. Actually, relative humidity is not directly measured. The air temperature and dew point temperature are measured, and the relative humidity is then calculated from those two values. The equation relating the three variables — temperature, dew point and relative humidity — is quite complex. Relative humidity is the ratio, expressed as a percentage, of the amount of water vapor in the air to the maximum amount it could hold at the temperature of the air. The complication is that as air is warmed, its capacity to hold water vapor increases dramatically. If no moisture is added, relative humidity drops as air warms.