Prevalence of wood stove use

In the United States, wood is the primary heating fuel for 2.25 million households and a secondary source of heating for nearly 9 million households. Residential wood stove use is highest in New England and the Mountain North regions, with individuals living in rural states with high forest covers such as Vermont (13.1%), Maine (8.6%), and Montana (6.5%) ^, reporting the greatest use of wood stoves as their primary household heat source per the 2022 US Census ( Fig. 2 ).

Estimated prevalence of ( A ) households using a wood stove as their primary heat source, ( B ) households using a private well for drinking water, and ( C ) land used for agriculture, for the 50 states in the United States. ^{, , ,}

Contaminants in wood stove smoke

Wood stove smoke contains a heterogeneous mixture of gaseous and particulate air pollutants resulting from the incomplete combustion of burned firewood. Wood smoke is primarily composed of particulate matter that is 2.5 μm or smaller in diameter (PM _2.5 ). When inhaled, PM _2.5 reaches deep into the alveoli and distal airways of the lungs. Wood stove smoke also contains other pollutants including trace elements, carbon monoxide, polycyclic aromatic hydrocarbons, and formaldehyde, among others ( Box 1 ). ^,

In northern New England, black carbon, a toxic component of PM _2.5 , is on average 62% higher in homes with versus without a wood stove. Black carbon is known to be a major pollutant in automobile exhaust, yet concentrations of black carbon in homes with a wood stove in northern New England were higher than levels reported in homes in nearby urban Boston, Massachusetts. Wood stoves are used primarily during winter months when children spend more time indoors, increasing exposure potential.

Health effects of wood stove smoke exposure

Reduced indoor air quality from wood stove smoke impacts an estimated 3 million children in the United States. ^, Robust evidence links PM _2.5 and black carbon exposure with respiratory outcomes such as wheezing and asthma during childhood. Greater black carbon exposure has also been associated with adverse cardiovascular health and premature mortality among children and adults. A recent study identified an association between wood stove use and incident lung cancer, adding evidence to the International Agency for Research on Cancer’s (IARC) characterization of indoor emissions from wood fuel as “probably carcinogenic” to humans. ^, Studies have also found that pregnant people exposed to wood stove smoke are more likely to have abnormal glycemia during pregnancy and small-for-gestational-age infants.

Prevalence of private wells

The US Environmental Protection Agency (EPA) estimated that more than 23 million US households acquired drinking water from private wells in 2010. Private wells are most prevalent in rural regions where access to municipal or public water utilities is unavailable. Areas with the highest density of private well use (>90 people/km ² ) are in the Northeast, around the Great Lakes and Southwest, and in areas scattered across the remaining United States (see Fig. 2 ). ^,

Public policies around well water testing

Although guidelines exist, most states do not regulate private well maintenance and testing. This stems from the 1974 Safe Drinking Water Act, which limits state and federal regulations to water systems with greater than 15 service connections or serving greater than 25 people, excluding Native American reservations. Thus, the onus falls on the homeowner to test their private well water to ensure it is safe for drinking. Many homeowners may not be aware of guidelines, despite recommendations by state health departments and other organizations such as the American Academy of Pediatrics (AAP). Data on the prevalence of private well testing are scant, but studies from New Jersey, Wisconsin, and New Hampshire indicate that few homeowners test their wells in accordance with guidelines, even when state testing regulations exist.

Contaminants in well water

Private well water contaminants depend on multiple factors including well depth and type (ie, dug versus drilled), geologic landscape, type of aquifer, and anthropogenic sources of contamination (eg, local industrial practices and land use). We list common contaminants in Box 2 . ^{, ,} Contaminants can be introduced into private wells through multiple sources, but industrial processes that produce surface water runoff/seepage and move into groundwater are a common pathway. For example, nitrates/nitrites, naturally occurring nitrogen-based molecules that can be harmful when consumed in excess, may be introduced into private wells via sewage, fertilizer, or animal waste. Sewage and animal waste are likewise pathways for pharmaceuticals to enter private wells. Industrial waste and plumbing/service lines can also leach contaminants into household water supplies. Perfluoroalkyl and polyfluoroalkyl substances (PFAS), endocrine-disrupting chemicals added to nonstick and oil-resistant consumer products, and pesticides can be introduced via products used in agricultural or industrial practices. Disinfection byproducts (DBPs) may contaminate private wells in communities where water is improperly chlorinated. Factors influencing contaminant levels include drought or flooding, proximity to industrial sites, and availability of testing and filtration systems. Well water contamination may also result in contamination of local food systems, with products such as locally produced eggs and produce and wild game and fish being further routes of exposure.

Health effects of contaminated well water exposure

Aside from the pathogenic effects of microbial contaminants, substantial epidemiologic evidence links private well water contaminants to adverse health impacts. Heavy metals and essential elements such as arsenic, chromium VI, uranium, and radon have been deemed Group I human carcinogens by IARC, while lead, selenium, and nitrates/nitrites are considered probable or possible human carcinogens. Beyond carcinogenicity, short-term, high-dose exposure to uranium can result in kidney damage, while long-term exposure to arsenic has been associated with increased risk of developing cardiovascular disease and diabetes. Exposure to both arsenic and lead can interfere with neurodevelopment and growth. Even exposure to essential elements at high levels can have toxic effects: for example, selenium has been associated with hypertension, and exposure to manganese with perturbations in neurodevelopment and growth. DBPs have been associated with reproductive toxicity.

Prevalence of biosolids

Biosolids are semi-solid materials formed after human and industrial wastes are treated physically and chemically at wastewater treatment plants. In the United States, over 15,000 wastewater treatment plants need to dispose of biosolids. Some biosolids are incinerated or put into landfills, but because of their high level of nitrogen, phosphorus, and organic carbon, over half of all biosolids are reused as fertilizer. ^, From 1998 to 2010, biosolids, referred to colloquially as “sludge,” were applied to over 1.2 million acres of farmland annually in the United States. While on the decline, this practice has continued with over 800,000 acres impacted in 2018. Many fields have over 20 years of biosolids application with the highest application density in the eastern United States. ^,

Routes of childhood exposure to biosolids

Children are typically exposed to biosolids via drinking water contamination; biosolids used as fertilizer seep into the ground and groundwater, where they contaminate residential drinking wells. ^, Other suspected pathways of exposure include consuming crops or meat and dairy raised on feed grown in soil fertilized with biosolids and consuming wild game or fish living in biosolid-contaminated areas.

Contaminants in biosolids

Biosolids used as fertilizer pose health risks because while the current sewage treatment process removes bacteria and other microorganisms, it cannot completely remove contaminants such as PFAS, microplastics, pharmaceuticals, surfactants, and heavy metals ( Box 3 ). These pollutants enter wastewater by being flushed down the toilet or excreted from the body via stool or urine. In addition, contaminants such as PFAS and microplastics in clothing and cleaning agents can enter wastewater from washing machines. Other contaminants, particularly heavy metals, enter wastewater through stormwater runoff, especially in areas where sewer and stormwater collection systems are combined.

Health effects of contaminated biosolids exposure

Each of the contaminants in biosolids has been independently associated with health risks. PFAS, which persist in the environment and have a long (3–8 year) biologic half-life in humans, ^, are associated with decreased antibody response to vaccines, dyslipidemia, increased kidney and testicular cancer risk, and lower birth weight. ^{, ,} The IARC has classified perfluorooctanoic acid as “carcinogenic to humans” and perfluorooctanesulfanoic acid as “possibly carcinogenic to humans.”

Microplastics, plastics less than 5000 μm in diameter, concentrate in the gastrointestinal tract, liver, kidney, and muscle and lead to oxidative stress, mitochondrial dysfunction, and inflammation, as well as metabolic toxicity and reproductive dysfunction in animal studies. Population-based studies in humans have suggested possible associations with inflammatory bowel disease and lung injury. ^{, ,} While microplastics are ubiquitous, newer techniques that are able to detect nanoplastics (<1 μm in diameter) have indicated that the presence of these contaminants in drinking water is likely orders of magnitude higher than previously reported for microplastics alone.

Pharmaceuticals such as antimicrobials and hormones are also found in biosolids, and exposure to these can promote the development of antibiotic-resistant bacteria and interfere with estrogen receptors and other reproductive and developmental pathways. ^,

Surfactants contain alkylphenol ethoxylates, a family of synthetic organic chemicals that accumulate in US biosolids in amounts that far exceed limits set for European countries. Surfactants and their metabolites have endocrine-disrupting properties and have been associated with nervous system anomalies. ^,

Heavy metals are present in biosolids, and their concentrations are largely regulated by the US EPA Part 503 Biosolids Rule. However, these regulations are not iron-clad, and in some cases, biosolids may be land applied against US EPA guidelines before heavy metal test results are available, and some metals/metalloids such as boron and silver, for which the health implications are not well understood, remain unregulated.

Prevalence of pesticides

Pesticides are widely used in US agriculture to increase crop yield and meet food demands. In 2012, more than 1 billion pounds of pesticides were applied in agricultural fields, with herbicides accounting for the largest share (63%), followed by fumigants (12%), fungicides (6%), and insecticides (4%) ( Box 4 ).

Routes of childhood exposure to pesticides

Widespread use of pesticides results in ubiquitous exposure among US children, largely from consumption of pesticide-contaminated food. However, children living in agricultural communities, where 90% of pesticides are currently used in the United States, are at particular risk. Children living on or near farmland are exposed to pesticides via drift from treated fields to nearby homes or schools ; take-home exposures (in which workers who have handled or been in contact with pesticides inadvertently carry substances into homes and vehicles on clothing or footwear) ; or consumption of contaminated water. Children living near areas sprayed with pesticides can also be exposed via direct contamination of their possessions, food, or water.

Health effects of pesticide exposure

Acute pesticide poisonings are relatively uncommon in US children, but exposure to pesticides has been linked to a broad range of chronic pediatric adverse health effects. Organophosphate (OP) pesticides, carbamates, and pyrethroids have been associated with cognitive and behavioral problems, including attention-deficit/hyperactivity and autism spectrum disorders. ^, Exposure to OP pesticides and pyrethroids in early life has been associated with poorer lung function and respiratory symptomatology in school-age children. ^, Home insecticide and herbicide use has been linked to childhood cancers such as leukemia, neuroblastoma, and brain cancer. Emerging literature suggests that exposure to current use pesticides, such as glyphosate, may be associated with obesity and cardiometabolic outcomes, and IARC designates these as “probably carcinogenic to humans.” ^,