Rainwater is safe for most household uses. Although some studies have shown that roof-harvested rainwater is generally potable, government agencies advise to avoid drinking the water unfiltered, as there are certain risks associated with it. Remember that rainwater comes in contact with various surfaces (roof, gutters, etc.) before it reaches your tank, not to mention the fact that it could already be carrying hazardous contaminants and pathogens such as the following:
Most pathogenic bacteria found in rainwater come from human and animal matter. Variables such as the specific type of bacteria and the strength of an individual’s immune system will determine if he or she will get sick from drinking untreated rainwater.