The University of Nebraska Extension services publish "NebGuides" on a variety of topics. During the Master Gardener classes your question was asked and answered this way (taken from the rain barrel NebGuide:
Rainwater collected in rain barrels is recommended for non-potable (water that is not drinking water quality and is not safe for consumption) uses, such as watering ornamental plants. Municipal drinking water often can contain sodium or other chemicals that can lead to plant health issues, while rainwater is free of these chemicals.
Rainwater from rain barrels is not recommended for watering fruit and vegetable gardens. It is not suitable for humans or pets to consume, or for uses that include direct body or food contact, such as in a child’s swimming pool or for bathing, hair washing, or washing produce.
Concerns with rain barrel water safety stem primarily from bird droppings and animal waste deposited on roof- tops. Bacteria and other disease-causing organisms could be picked up by rainwater flowing over rooftops.
Another safety concern are chemicals in roof runoff that could come from asphalt roofing, redwood or cedar shakes, treated wood shingles, roofing materials contain- ing asbestos, lead solder or lead-based paints, metals such as zinc from galvanized flashing, moss killers, and other sources. Deposition of industrial pollutants and/or herbicide drift on rooftops also may be a potential issue depending on where you live.
While bacteria and chemical deposition are not con- sidered major concerns for non-potable uses of collected rainwater, they provide sufficient reasons not to use rain barrel water for potable uses. Specialized filtering and treat- ment are required for rainwater to be suitable for drinking, cooking, bathing, or other direct human contact.