Since rain may be exposed to contaminants in the air or in the method of collection, it may be too "dirty" to drink. Rainwater is not generally suitable for use as drinking water without testing and possible treatment.
Pollution, such as soot, smoke and dust, may be incorporated into rainwater as it falls through the atmosphere. In agricultural areas, airborne fertilizer residues may contribute a higher concentration of nitrates in rainwater. The method of collecting rainwater can wash molds, algae, bacteria and other organic matter into storage containers. It can be difficult to harvest a significant amount of rainwater safely without risking at least some bird droppings winding up in the rain barrel.
Harvesting rainwater for potable use is possible, however, and the first step is to test the water to make sure it can be easily treated. Rainwater harvesting systems are available that provide guidelines and equipment for safe collection, filtration and treatment of rainwater for drinking.