Of all known liquids, water comes closest to a universal solvent; water dissolves more substances than any other known substance. That tendency to dissolve materials also means that water has minerals, oxygen, chemicals and bacteria swirling around within it. The safety of rain water then depends on what impurities it may contain or carry.
TL;DR (Too Long; Didn't Read)
The safety of drinking rainwater depends on the cleanliness of the atmosphere the water vapor has passed through. How the rain is collected also impacts the quality of the water. If the rain is collected directly from the air in a relatively remote area without upwind sources of air pollution, and then is boiled to kill bacteria, rainwater might be safe to drink.
Water Cycle Review
The water cycle, while very complex in its details, can be generalized as having three steps: evaporation, condensation and precipitation. Evaporation occurs when water molecules gain enough energy to become water vapor. The energy usually consists of heat energy from the sun, but chemical reactions ranging from plant and animal respiration to internal combustion engines and factory emissions also release water vapor into the atmosphere.
Water vapor floats in the atmosphere, eventually clumping together with other water molecules. Often this clumping occurs around another floating particle. These particles may be from chemicals, dust, soot, bacteria or pollen. Condensation happens when the water vapor becomes liquid again.
When the water droplets become large enough to fall, precipitation begins. Precipitation can be in the form of rain, snow, hail or a combination. The water returned to the Earth's surface may sink into the ground; run off into rivers, streams, lakes or the ocean; be absorbed by plants; drunk by animals; or used by industry, but sooner or later water evaporates and the cycle continues.
One of the benefits of rainwater harvesting is the volume available. For example, 1 inch of rain falling on a structure with a 40-foot by 70-foot roof area yields about 1,700 gallons (6,600 liters) of water. The water can be captured by rain barrels or cisterns attached to the downspouts. If the first runoff is diverted to the ground, at least some of the accumulated debris, dust, bacteria and other contaminants will be washed away. The remainder might be safe, at least for irrigation of non-food plants and raingardens and, if relatively clean, for wildlife water sources. Using harvested rainwater reduces the amount of treated water from public systems, conserving water.
Many states have legislation controlling or prohibiting rainwater harvesting. Colorado, for example, in 2016 enacted rules that limit private homeowners to two rain barrels (110 gallons) of harvested rainwater. The water must be used on the property for outdoor purposes like garden and landscape irrigation. In Oregon, rainwater harvesting is allowed, but it can only be collected from roof surfaces. Homeowners should check their state's regulations before installing a rainwater harvesting system.
The quality of rainwater varies greatly from place to place, depending on the types of contaminants and distance from contamination sources. For example, taller smokestacks partially relieved London's smog issues by spreading the contaminated smoke across wider areas. Studies show that rainwater in air pollution centers like Los Angeles will contain chemical contaminants.
Regulations vary from state to state for the use of harvested rainwater for drinking water. If maintained for private use, many states do not enforce drinking water standards, leaving the responsibility with the homeowner. For safety, however, homeowners should have their water tested before using rainwater for drinking water. The U.S. Environmental Protection Agency issued updated the drinking water standards and health advisories in 2018 (see Resources).
Cleanliness of Rainwater
Rain falling through the atmosphere would seem like the cleanest water on Earth. Unfortunately, water's ability to carry so many different dissolved or suspended materials makes this an unsafe assumption. Even if the rainwater is relatively pure, the collection method impacts the cleanliness of rainwater. Stored rainwater can also become contaminated.
Potential Contaminants in Rain
Airborne materials may become dissolved or suspended in rain drops, contaminating rainwater. For example, air monitoring in the Los Angeles area between 1995 and 1998 showed residents were exposed to nearly five times the recommended levels of the carcinogenic compounds benzene, formaldehyde and butadiene. These chemicals were carried from the atmosphere to the ground during rain storms.
Sulfates and nitrogen oxides from air pollution chemically combine with water droplets to form acid rain. Rainwater naturally has a pH of 5 to 6, which is slightly acidic. Acid rain, however, may reach a pH as low as 2, but usually has a pH of about 4. Although the lowest acid rain pH of 2 equals the pH of vinegar (2.2) and lemon juice (2.3), acid rain isn't directly harmful to drink. The direct harm to humans (and other animals) comes from breathing the acid rain. When rain falls or smog forms, the relative humidity of the atmosphere is 99 to 100 percent. At this point, breathing brings the acid material into the lungs. People with asthma, respiratory disease or impaired respiratory function are particularly at risk.
The Great London Fog of 1952 directly killed an estimated 4,000 people, with total deaths estimated between 8,000 and 12,000 because of the five-day-long acid smog event. In 1966, the Thanksgiving weekend smog event caused about 200 people deaths in New York City. In the 1960s smoke and smog-related deaths due to bronchitis and pulmonary emphysema were increasingly common in New York City.
Bacteria in Rainwater
Rainwater collected from rooftops likely contains bacteria from bird droppings, small-mammal scat and organic decomposition. An Australian study showed that airborne bacteria add significantly to this bacterial load.
Rainwater may be better for plants because it lacks the chemicals of public water treatment facilities. However, harvested rainwater is not recommended for watering food crops. If the water is used for watering fruits and vegetables, the water should not be applied directly to the plant. Apply the possibly contaminated water to the soil around the plant in the early morning and delay harvesting until later in the day when evaporation and ultraviolet exposure should kill any bacteria. Treating rainwater with bleach or iodine on the assumption the water is contaminated by bacteria is also suggested.
Dust, Dirt, Smoke and Pollen
Dust, dirt, smoke and pollen picked up by wind, cars, mowing, fires and other activities become part of the atmosphere. Water vapor condenses around the particles. The dust, dirt, smoke and pollen return to the ground with rain. These materials deposited on roofs wash off during rainstorms, especially during the first storms after a dry spell. These natural materials can pose health issues in rainwater.
When rain washes across a roof, particles of the roof and gutter materials join the dust, soot, pollen and airborne chemicals in the runoff. Construction materials like asbestos, asphalt (a petroleum product) and metals (lead and copper) may contaminate the runoff.
Contamination of Stored Rainwater
Collected rainwater should be used within 10 days to prevent mosquito larva infestation. Screens should be used to prevent debris and animal contamination from entering the storage container. Cisterns with welds may become contaminated with lead from the solder. Treating the rainwater with bleach or iodine won't remove chemical contaminants.
- U.S. Geological Survey: Water, the Universal Solvent
- NASA: Precipitation Education – The Water Cycle
- North Carolina State University: What Is Acid Rain?
- U.S. Environmental Protection Agency: What Is Acid Rain?
- Stanford University: Smog
- Encyclopaedia Britannica: The Great Smog of London
- The New York Times: Remembering a City Where the Smog Could Kill
- Water Research: Wind, Rain and Bacteria – The Effect of Weather on the Microbial Composition of Roof-Harvested Rainwater
- Rutgers: New Jersey Agricultural Experiment Station: Rain Barrels Part IV: Testing and Applying Harvested Water to Irrigate a Vegetable Garden
- Centers for Disease Control and Prevention: Rainwater Collection
- Texas A&M AgriLife Extension Service: Rainwater Harvesting
- National Conference of State Legislatures: State Rainwater Harvesting Laws and Legislation