Rainwater is not pure water. Even before a single drop touches the ground, it carries dissolved gases, mineral dust, sea salt ions, living microbes, and in the modern world, synthetic chemicals like PFAS and microplastics. The exact cocktail depends on where the rain forms, what the air below it contains, and what surfaces it contacts on its way down. What looks like clean water falling from the sky is actually one of the atmosphere’s most effective delivery systems for both natural and human-made substances.
The Natural Chemistry of a Raindrop
Every raindrop begins as water vapor that condenses around a tiny particle, called a condensation nucleus. That particle might be a grain of mineral dust, a salt crystal from ocean spray, a volcanic ash fragment, or even a bacterium. The chemistry of the resulting raindrop reflects these origins. Studies of precipitation chemistry consistently find dissolved ions that trace back to sea salt and crustal dust as dominant natural sources. In coastal and monsoon-influenced regions, sodium and chloride ions dominate, carried inland by marine aerosols. Research in India’s Western Ghats found that sea salt and long-range transported mineral dust accounted for the majority of ions like sodium, chloride, calcium, and magnesium in rainwater.1Scientific Reports. Influence of dust and sea-salt sandwich effect on precipitation chemistry over the Western Ghats during summer monsoon Even at inland sites hundreds of kilometers from the coast, the chloride-to-sodium ratio in rainwater can closely match that of seawater, indicating that marine aerosols travel vast distances before being washed out.2PubMed. Chemical composition of major ions in rainwater
Carbon dioxide dissolved from the atmosphere makes even the cleanest rain slightly acidic, with a pH around 5.6. But natural alkaline dust can push the pH back up. In northern China, for instance, calcium carbonate from desert dust neutralizes sulfuric acid in precipitation, keeping pH relatively high despite heavy sulfur emissions from industry.3Environmental Pollution. Acid rain and acidification in China: the importance of base cation deposition This neutralizing effect means that two regions with identical pollution levels can have very different rainwater acidity, depending on the local dust composition.
Living Passengers in the Clouds
Some of the most surprising things in rainwater are alive. Certain species of bacteria and fungi act as biological ice nuclei, triggering ice crystal formation in clouds at temperatures well above what inorganic particles require. These organisms can initiate freezing at temperatures between about minus 2 and minus 10 degrees Celsius, while mineral dust typically needs temperatures below minus 15 to do the same job.4Atmosphere. Biological Ice Nucleators in Snow Samples from Greece Researchers have identified and measured ice-nucleation-active bacteria in both precipitation and on plant surfaces, though how much these microbes actually contribute to triggering rainfall on a global scale remains an open question.5PubMed Central. Measurement of ice nucleation-active bacteria on plants and in precipitation by quantitative PCR
Beyond bacteria, rainwater can carry fungal spores, pollen fragments, and other biological debris. These bioaerosols are swept into the atmosphere from soil, vegetation, and bodies of water, then incorporated into cloud droplets or scavenged by falling rain. For most people, the biological content of rain is harmless. But it becomes relevant in specific scenarios like thunderstorm asthma, covered later in this article.
Heavy Metals from Traffic and Industry
Urban rainwater picks up heavy metals that have accumulated in the lower atmosphere from vehicle exhaust, tire wear, brake dust, and industrial emissions. Zinc tends to show up at the highest deposition rates among heavy metals, and its concentration in rain correlates with traffic volume. Lead, cadmium, nickel, and copper, by contrast, are more closely linked to traffic congestion, where stop-and-go driving generates more brake and clutch wear.6Atmospheric Environment. Atmospheric deposition as a source of heavy metals in urban stormwater These metals attach to airborne particles of different sizes: zinc tends to ride on larger particles (above 10 micrometers), while lead, cadmium, nickel, and copper cling to finer particles under 10 micrometers. The finer particles stay airborne longer and travel farther, which means these metals can show up in rain well downwind of their source.
Rainfall depth matters too. Heavier storms wash out more of these fine particles, so bulk deposition (wet plus dry combined) increases with rainfall depth. This is part of a broader pattern where rain acts as the atmosphere’s cleaning mechanism, pulling pollutants from the air column and depositing them on the ground.
PFAS, the “Forever Chemicals”
Per- and polyfluoroalkyl substances, known as PFAS, are a family of synthetic chemicals used in nonstick coatings, water-resistant fabrics, firefighting foams, and food packaging. They earned the nickname “forever chemicals” because their carbon-fluorine bonds are extraordinarily stable and resist natural breakdown. PFAS have become so widespread in the atmosphere that they now appear in rain almost everywhere researchers look, including remote locations far from obvious sources.
A review of five recent studies on North American precipitation found average total PFAS concentrations ranging from roughly 2 to 92 nanograms per liter. On average, these levels fell below the maximum contaminant levels recommended by the U.S. EPA, with hazard index values between 0.05 and 0.30. However, rainwater collected near known point sources like military bases, airports, or industrial facilities often exceeded those guidelines.7Water Research. Contamination levels of per- and polyfluoroalkyl substances (PFAS) in recent North American precipitation events. A review. The distinction between background rain and rain near a source matters a great deal: someone harvesting rainwater in a rural area far from PFAS-emitting industry faces a very different exposure profile than someone near an airfield where aqueous film-forming foam has been used.
Microplastics Falling from the Sky
Plastic particles smaller than five millimeters, classified as microplastics, are now a routine finding in rainwater samples worldwide. They enter the atmosphere through the breakdown of larger plastic waste, tire abrasion on roads, synthetic textile fibers released during laundering, and industrial emissions. Once airborne, they can travel long distances before being scavenged by rain or snow.
Studies in different parts of the world paint a consistent picture: microplastics in rain are ubiquitous. Research in Bahia Blanca, Argentina, found total anthropogenic debris in atmospheric deposition samples averaging about 77 particles per sample, with deposition fluxes around 100 items per square meter per day.8PubMed Central. Microplastics and anthropogenic debris in rainwater from Bahia Blanca, Argentina In Wroclaw, Poland, researchers found an average of about 135 suspected microplastic particles per liter in residential areas and 168 per liter near traffic-heavy zones, with fibers being the dominant shape.9PubMed. Quantifying morphological complexity and wet deposition of suspected microplastics in rainwater: A case study of Wroclaw, Poland The health implications of inhaling or ingesting microplastics at these concentrations are still being studied, but their sheer prevalence in precipitation underscores how deeply synthetic materials have penetrated Earth’s water cycle.
Pesticides That Evaporate and Return
Pesticides applied to crops don’t all stay in the field. A fraction evaporates into the atmosphere, sometimes within hours of application, and can be carried by wind before being washed back down with rain. This volatilization-and-redeposition cycle has been documented extensively in agricultural regions.
On the Delmarva Peninsula in the eastern United States, event-based precipitation sampling over four years found that herbicides dominated pesticide wet deposition, accounting for roughly half to two-thirds of the total. The fungicide chlorothalonil was the single largest contributor by mass. The timing of rainfall relative to pesticide application turned out to be a key factor: rain shortly after spraying produced much higher concentrations.10PubMed. Wet deposition of current use pesticides at a rural location on the Delmarva Peninsula: impact of rainfall patterns and agricultural activity Similar patterns have been found in Brazil, where researchers detected pesticides in rainwater at concentrations ranging from 0.1 to nearly 600 nanograms per liter across cities with varying land use. Atrazine and tebuconazole were found in every single sample, and deposition fluxes in agricultural areas were about three times higher than in the major urban center of São Paulo.11PubMed. Pesticides in rainwater: A two-year occurrence study in an unexplored environmental compartment in regions with different land use in the State of São Paulo – Brazil
This matters ecologically because wet deposition can deliver pesticides to waterways and ecosystems that were never targeted by spraying. Research in the southeastern U.S. Coastal Plain confirmed that localized pesticide wet deposition can present ecological risks, and that volatilization followed by rain-out is a meaningful transport pathway at local scales.12PubMed. Assessing pesticide wet deposition risk within a small agricultural watershed in the Southeastern Coastal Plain (USA)
Black Carbon and Soot
Black carbon, the sooty material produced by incomplete combustion of fossil fuels and biomass, is another regular passenger in rain. Rain is one of the primary mechanisms that removes black carbon from the atmosphere through wet deposition. The amount that comes down varies dramatically by region, driven by both the concentration of soot in the air and local rainfall patterns. Over China’s Sichuan Basin, which experiences both high industrial emissions and substantial rainfall, researchers found the highest black carbon wet deposition flux in the country. The analysis showed that the density of black carbon in the atmospheric column mattered more than the amount of rainfall in determining how much ended up on the ground.13Atmosphere. High Wet Deposition of Black Carbon over the Sichuan Basin of China
Nitrogen Compounds and Ecosystem Effects
Dissolved nitrogen, in both organic and inorganic forms, is one of the most ecologically significant components of rainwater. Nitrogen oxides from vehicle exhaust, power plants, and industrial processes combine with water vapor to form nitric acid, while ammonia from agriculture reacts in the atmosphere to produce ammonium salts. Both end up in rain.
When this nitrogen-laden rain reaches the surface, it acts as unintended fertilizer. That sounds benign, but excess nitrogen deposition drives water eutrophication, where algal blooms choke aquatic ecosystems, and can reduce biodiversity in sensitive habitats.14Environmental Technology & Innovation. Rainwater records of atmospheric nitrogen-bearing pollutants in a plateau city, China: Compositions, evolution, and sources Research in Nigeria’s Rivers State confirmed that both dry and wet deposition of nitrogen substantially affect water quality, with atmospheric aerosols and biogeochemical interactions influencing concentrations across different land surfaces.15PubMed. Chemometric and risk assessment of nitrogen composition of atmospheric rainwater from diverse surfaces in Rivers State, Nigeria
Volcanic Eruptions and Extreme Chemistry
Volcanic activity can radically alter rainwater composition over wide areas. During Iceland’s 2014–2015 Holuhraun eruption, which released an estimated 11 megatons of sulfur dioxide along with hydrogen chloride and hydrogen fluoride, precipitation chemistry across the entire island shifted dramatically. Fluoride and sulfate concentrations in rainwater reached levels roughly 20 times higher than pre-eruption averages, with sulfate values peaking above 17,000 micromoles.16Journal of Geophysical Research: Atmospheres. Major impact of volcanic gases on the chemical composition of precipitation in Iceland during the 2014–2015 Holuhraun eruption These are extreme conditions, but they illustrate how volcanic degassing can overpower the normal atmospheric chemistry of rain in a matter of days. High fluoride in rainwater is especially concerning because it can damage livestock teeth and bones, contaminate surface water supplies, and harm vegetation.
The Seasonal First Flush
Not all rainstorms deliver the same pollutant load. The first significant rains after a dry spell tend to carry much higher concentrations of contaminants than later storms. This pattern, called a seasonal first flush, occurs because pollutants accumulate in the atmosphere and on surfaces during dry periods, then get swept out in the initial rains. Research on urban stormwater in California found that pollutant concentrations in the first storms of the wet season were anywhere from 1.2 to 20 times higher than concentrations later in the season. The effect was strongest for organic pollutants, dissolved minerals, and heavy metals other than lead.17PubMed. Seasonal first flush phenomenon of urban stormwater discharges If you collect rainwater for any purpose, the practical takeaway is straightforward: diverting the first flush of a storm or the first rains of the season can meaningfully reduce the contaminant load in your collection.
Cosmogenic Traces and Radioactivity
Rainwater carries faint traces of radioactive material, but not the kind that should alarm you. Beryllium-7 is a radionuclide created naturally in the upper atmosphere when cosmic rays collide with nitrogen and oxygen atoms. Once formed, it attaches to fine aerosol particles and is eventually washed to the ground by rain.18PubMed. Cosmogenic beryllium-7 in soil, rainwater and selected plant species to evaluate the vegetal interception of atmospheric fine particulate matter The amounts are vanishingly small and pose no health risk. Scientists value beryllium-7 precisely because it is so reliable: since it is produced at a relatively constant rate and carried down by rain, it serves as a natural tracer for studying atmospheric circulation patterns, tropopause dynamics, and even the movement of jet streams.19Scientific Reports. Radioisotopes demonstrate changes in global atmospheric circulation possibly caused by global warming Researchers have used beryllium isotope ratios in rainwater as a tool for atmospheric transport analysis since the 1960s.20Journal of Nuclear Physics, Material Sciences, Radiation and Applications. Preliminary Measurements of Be-10/Be-7 Ratio in Rainwater for Atmospheric Transport Analysis
Thunderstorms, Pollen, and Asthma
Rain normally suppresses airborne pollen by washing it out of the air. But thunderstorms can reverse this relationship in a dramatic and dangerous way. Outflow winds from a thunderstorm concentrate pollen grains at ground level, and the combination of high humidity and rainfall causes the grains to rupture through osmotic shock. This releases tiny allergenic particles small enough to penetrate deep into the lungs.21PubMed Central. Thunderstorm-triggered asthma: what we know so far The phenomenon, called epidemic thunderstorm asthma, has triggered mass-casualty events. During Melbourne’s 2016 episode, the world’s deadliest recorded thunderstorm asthma event, counts of ruptured grass pollen grains increased by 250 percent when the storm outflow reached monitoring stations. The surge correlated with increased particulate matter, high humidity, and dropping temperatures.22PubMed Central. The perfect storm: temporal analysis of air during the world’s most deadly epidemic thunderstorm asthma (ETSA) event in Melbourne So while rain itself does not typically deliver allergens in a harmful way, the particular meteorological context of a thunderstorm can turn pollen-laden rain into a genuine respiratory hazard for sensitized individuals.
Microbial Risks in Harvested Rainwater
Freshly fallen rain contains relatively few pathogens. The problem is what happens after it lands. Roof-harvested rainwater, collected through gutters and stored in tanks, picks up bacteria from bird droppings, decaying leaves, insects, and biofilms on roofing materials. Studies consistently show that stored rainwater frequently exceeds drinking water standards for microbial indicators.
In Adelaide, Australia, more than half of 53 household rainwater tanks tested positive for E. coli, and samples from about a fifth of them exceeded even the less stringent limits set for recreational water quality.23PubMed Central. Microbiological Values of Rainwater Harvested in Adelaide A larger study across four communities in the southwestern United States tested nearly 600 harvested rainwater samples and found that the majority exceeded U.S. EPA safe drinking water limits for total coliforms, while E. coli exceedances were less common, ranging from about 4 to 15 percent of samples depending on location.24Journal of Applied Microbiology. Assessing the impact of rainwater harvesting infrastructure and gardening trends on microbial indicator organism presence in harvested rainwater and garden soils
More concerning is the detection of specific human pathogens. In a South African study of roof-harvested rainwater, Listeria monocytogenes and Mycobacterium tuberculosis were detected in every sample tested, while Yersinia species appeared in 92 percent of samples.25PubMed Central. Human Pathogenic Bacteria Detected in Rainwater: Risk Assessment and Correlation to Microbial Source Tracking Markers and Traditional Indicators Detection by molecular methods does not necessarily mean the organisms were viable or present at infectious doses, but it reinforces that untreated harvested rainwater is not equivalent to treated drinking water. If you use collected rainwater for drinking or cooking, some form of treatment, whether filtration, UV disinfection, or chlorination, is a reasonable precaution.