Planting Healthier Indoor Air - PMC

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Poor indoor air quality has been linked to health problems, especially in children. Asthma has reached epidemic proportions among multiple age groups and is considered the most common chronic disease in urban-dwelling children.1 The American Academy of Allergy, Asthma and Immunology Indoor Allergen Committee suggested in a 2010 report that allergists consider indoor air filtration to be part of a comprehensive strategy to improve respiratory health.2 Air cleaners with HEPA filters have been shown to improve symptoms of asthma.2 However, filtration systems and air purifiers do not reduce levels of all indoor air pollutants, and some types can actually aggravate the problem. For example, one study showed that some air purifiers raise indoor concentrations of ozone above safety levels established by the U.S. Environmental Protection Agency.3

A more benign addition to air filtration could be the use of houseplants. In addition to basic photosynthesis that removes carbon dioxide and returns oxygen to the air, plants can remove toxicants from air, soil, and water in at least two ways. First, they can metabolize some toxic chemicals, releasing harmless by-products, and second, they can incorporate toxicants such as heavy metals into plant tissues, thus sequestering them.

Data on plant-mediated indoor air quality come from experiments conducted by the U.S. National Aeronautics and Space Administration (NASA). As NASA researchers explored the possibilities of long-term space habitation, it became evident that the air in a tightly sealed space capsule would quickly become contaminated with volatile organic compounds (VOCs) and other chemicals released by the materials used to manufacture the capsule interior.4

This is similar to the situation in newly constructed energy-efficient dwellings. If energy-efficient construction is not carefully designed to maintain indoor–outdoor air exchange, one unintended consequence can be increased concentrations of pollutants indoors. For example, in a study recently published in the American Journal of Public Health, Gary Adamkiewicz and colleagues used a simulation model to demonstrate that in homes with low air exchange rates and multiple sources of air pollution, up to 90% of exposure to fine particulate matter came from indoor sources.5 Besides particles and VOCs, indoor air and dust can also contain brominated flame retardants, pesticides, toxic metals, and other pollutants.6

For more than 30 years, B.C. “Bill” Wolverton, a retired civilian scientist for NASA, investigated the use of plants as air- and water-purifying systems for enclosed environments in space missions. Through his research, Wolverton found the air-cleaning capacity of houseplants can be improved exponentially by increasing air circulation to the roots of the plants, where symbiotic microorganisms help make the substances culled from air bioavailable to the plant.

In those studies, Wolverton and colleagues tested several types of low-light houseplants.7 For example, golden pothos (Epipremnum aureum, also known as devil’s ivy) grown on an activated carbon filter system reduced air levels of benzene and trichloroethylene inside a Plexiglas chamber measuring 0.58 cubic yard from approximately 36 ppm to barely detectable levels within 2 hours.4 Experiments conducted elsewhere by Stanley J. Kays and colleagues at the University of Georgia also documented the ability of different plant species to remove VOCs such as benzene, toluene, octane, and trichloroethylene.8

One indoor contaminant of particular concern is formaldehyde, which is released by many household products, among them pressed woods, some types of foam insulation, paper products, some paints and varnishes, and permanent-press fabrics. The National Toxicology Program lists formaldehyde as reasonably anticipated to be a human carcinogen.9

In an unpublished 2006 study, Wolverton tested a small fan-assisted planter/air filter inside a travel trailer that had been used as temporary housing for displaced Hurricane Katrina victims. This trailer, like similar units, had been found to be highly contaminated with formaldehyde. The plant/air filter contained a plant growing in a mixture of activated carbon and expanded clay pebbles. Wolverton’s tests showed that the levels of formaldehyde were reduced from potentially toxic levels of 0.18 ppm to 0.03 ppm, within the safety limits defined by the World Health Organization.10

Those studies fit well with evidence on the biochemical mechanisms involved in plant detoxification of formaldehyde. In studies published this year Zhongjun Xu and colleagues tested three kinds of potted plants for their capacity to remove formaldehyde from indoor air in test chambers. They found that the formaldehyde-removal capacity of the plants depended on the dehydrogenase activity in the leaves and root system—that is, how efficiently the plant could metabolize formaldehyde.11 As Wolverton found earlier, these investigators also found that formaldehyde removal by plants was diffusion-limited. That means increasing the circulation of contaminated air through the root system and leaves improved the formaldehyde-removal effect.

In another recent study, Kays and colleagues tested 86 species of houseplants from five general classes for their ability to remove formaldehyde. In their experiments, ferns had the highest formaldehyde-removal efficiency of all the plants tested, especially Osmunda japonica, commonly known as Japanese royal fern, or zenmai.12

Another important air contaminant that is amenable to plants’ cleanup abilities is mercury vapor. Mercury can make its way into homes through accidental spills (for instance, breakage of thermometers and fluorescent bulbs) as well as through its use in certain cultural and religious practices.13 Mercury vapor is neurotoxic and lingers in the air even after new sources have been eliminated from the environment.14

Joao Paulo Machado Torres, a senior scientist at the Radioisotopes Laboratory of the Federal University of Rio de Janeiro, Brazil, and his group have published many studies on the use of plants in indoor and outdoor mercury-contaminated settings.15 “We have used plants of the bromeliad family and Spanish moss (Tillandsia usneoides) as sentinel species to detect and absorb mercury from the air in shops contaminated by the gold trade in the Amazon,” he says. The use of plants can be uniquely useful in these environments where other kinds of remediation technology may be impractical or difficult to deploy.

But as has been shown with many natural remedies, “natural” does not necessarily equate to “absolutely harmless.” A study by Kays and colleagues published in 2009 pointed out that some houseplants—as well as the media and plastic pots they are grown in, the microorganisms that inhabit them, and the pesticides used to treat them—can potentially contaminate indoor air with VOCs.16 “It is not yet possible to project the true potential of plants for purifying indoor air,” Kays says. “At this time the role of plants, though appearing [generally] positive, is not totally clear. The absence of funding for phytoremediation research has greatly impeded solving the problem.”

Kays also notes the lack of an accurate means for the public to determine if the VOCs in their home or office represent a significant health problem. “The absence of a relatively inexpensive method available to the public results in situations where it takes two and a half years to determine that the Katrina trailers had toxic levels of formaldehyde even though there had been health complaints by the occupants almost as soon as the trailers were in place,” he says. “If an accurate, reasonably priced method was available from a credible source such as a university extension analytical laboratory, the public would be able to ascertain their potential health risk before buying or renting a house, apartment, or office.”

General Information for Formaldehyde

Chemical Group: N/A

CASRN: 50-00-0

Risk Evaluation Initiated: December 2019

Docket: EPA-HQ-OPPT-2018-0438

Staff Contact: Jeffrey Putt
Putt.jeffrey@epa.gov
(202) 564-3703

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On March 15, 2024, EPA released the draft risk evaluation under the Toxic Substances Control Act (TSCA) for formaldehyde for public comment and peer review. 

Formaldehyde’s use as a pesticide is also undergoing a separate review under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA). The FIFRA risk assessment, which addresses the pesticidal uses of formaldehyde, will be released in 2024 on docket EPA-HQ-OPP-2015-0739.

Find other information about other chemicals undergoing risk evaluations under TSCA. 

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Risk Evaluation Findings

EPA preliminarily finds that formaldehyde poses unreasonable risk to human health. However, EPA notes that these risks may not apply to everyone, everywhere and describes some of the sources of uncertainties in EPA’s findings.

Due to its varied sources, people are routinely exposed to formaldehyde in indoor and outdoor environments, often from more than one source at a time. High levels of exposures to formaldehyde can cause health problems when inhaled and if it is absorbed into the skin. Inhaling high levels of formaldehyde for a short period of time can cause sensory irritation such as eye irritation. Inhaling formaldehyde for longer periods of time can damage the lungs and increase asthma and allergy-related conditions, sensory irritation, reproductive toxicity, and cancer. Skin contact with products containing formaldehyde can also cause allergic reactions.

In the draft risk evaluation, EPA evaluated the risks that arise from ways in which people may be exposed to formaldehyde from the production and use of products that are subject to TSCA - as opposed to exposures from those products that are excluded from TSCA (such as pesticides and exposures from sources of formaldehyde that are biogenic such as breathing and the decomposition of leaves) and exposures from other sources of formaldehyde.

EPA found that workers who are in workplaces where formaldehyde is used are at the most risk from formaldehyde exposure. Workers may be exposed to formaldehyde by inhaling it after it is released into the air or by making skin contact with formaldehyde-containing materials. EPA made these conclusions without assuming that worker protections, such as wearing protective equipment, were in place, although EPA is aware that many employers do take measures to protect the safety of their workers.

EPA also found that people who frequently use certain consumer products that contain formaldehyde are at risk. These products included car waxes, some crafting supplies, and fabrics or leather goods treated with formaldehyde. However, a person’s risk from these products depends on how long and how frequently the products are used, and in many cases, exposures from these products are at the same or lower levels than exposures from other sources of formaldehyde in the home such as candles or cooking. People may also be exposed to more formaldehyde in their homes if the homes are newer or contain newer furniture because building products and furniture can release formaldehyde at higher levels when newly formulated. Lastly, people living near facilities releasing formaldehyde may have higher exposures than those located further away.

EPA also evaluated formaldehyde’s impact on the environment and found that formaldehyde is not expected to last long in water, sediment, or soil based on its physical and chemical properties. EPA did not identify risk of injury to the environment that would contribute to the unreasonable risk determination for formaldehyde.

Background on Formaldehyde

Formaldehyde is a colorless, flammable gas at room temperature and has a strong odor. Formaldehyde is found nearly everywhere. People and animals produce and release formaldehyde. Formaldehyde is produced when organic material including leaves, plants, and woodchips decay. Formaldehyde is also produced and released into the air when things burn, such as when cars emit exhaust, when furnaces and stoves operate, and through forest fires, burning candles, and smoking. Finally, formaldehyde is used to make many products including composite wood products and other building materials, plastics, pesticides, paints, adhesives, and sealants. Information from the 2016 Chemical Data Reporting (CDR) for formaldehyde indicates that the reported production volume is between 1 billion and 5 billion lbs/year (manufacture and import).

Uses of Formaldehyde

In the final scope, EPA identified conditions of use associated with the importing; processing; distribution in commerce; industrial, commercial and consumer uses; and disposal of formaldehyde, for example:

• Adhesive and sealants;
• Pesticides and agricultural chemicals;
• Lawn and garden products;
• Personal care products;
• Plastic materials;
• Resins, glues, and binders;
• Paper manufacturing;
• Various petrochemical processes and products;
• Wood product manufacturing; and
• Textiles, apparel, and leather.

The above listed conditions of use are ways that a person or the environment could be potentially exposed to this chemical. However, when conducting a risk evaluation, EPA also considers the hazards (i.e. health effects or environmental impacts) that could occur from coming in contact with a chemical.

Recent Activities and Opportunities for Public Comment

On March 15, 2024, EPA released the draft risk evaluation for this chemical for public comment and peer review. EPA will accept public comments on the draft risk evaluation for 60 days following publication in the Federal Register via docket EPA-HQ-OPPT-2023-0613 at regulations.gov.

EPA will hold a virtual peer review public meeting for of the Science Advisory Committee on Chemicals (SACC) to discuss the draft risk evaluation from May 20-23, 2024. EPA will also hold a virtual preparatory public meeting for the SACC and the public to consider and ask questions regarding the scope and clarity of the draft charge questions on May 7, 2024. View the SACC website for more information on the May 2024 meetings.

In August 2020, EPA published a final scope document outlining the hazards, exposures, conditions of use, and the potentially exposed or susceptible subpopulations the agency expects to consider in its risk evaluation. The agency released the draft scope in April 2020 and took public comments on the draft document.

As EPA continues to move through the risk evaluation process there will be additional opportunities for public comment, including a public comment period on the draft risk evaluation. In addition to public comment periods, EPA will continue to engage with stakeholders as it refines the risk evaluation and stakeholders should reach out to EPA via the staff contact to engage with the agency.

Draft Risk Evaluations and Supporting Documents

The draft risk evaluation has a different structure compared to previous risk evaluations released by EPA. Rather than one document, the risk evaluation for formaldehyde includes several documents that culminate in an environmental risk assessment, human health risk assessment, and unreasonable risk determination. 

Read the Federal Register notice.

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