Month by month, as scientists learn more about the effects of a deadly tire-related chemical, U.S. tire manufacturers are finding themselves under increasing pressure to come up with rubber formulations less toxic to fish that do not compromise automotive safety.
A lawsuit filed against 13 of the largest tire manufacturers in this country seeks to ban the chemical 6PPD from its universal use in tires, or else force tire companies to pay for stormwater-mitigation measures that can remove the chemical or otherwise protect salmon. The lawsuit alleges violations of the federal Endangered Species Act, which is designed to prevent extinction of threatened and endangered species.
Elizabeth Forsyth, attorney for Earthjustice in Seattle who filed the lawsuit on behalf of two fishing groups, said the law clearly prohibits harm to listed species unless the federal government approves an “incidental take permit.”
“By having a super-toxic chemical in the tires, you are harming the fish,” Forsyth said. “The Endangered Species Act doesn’t say that you can’t do that (harm fish), but you can’t do that without a permit.”
Starting in the 1950s, 6PPD has been infused in the rubber of tires to prevent degradation caused by ozone. Without the additive, tires are known to crack and degrade much more rapidly.
Coho salmon, Chinook salmon and steelhead trout have been shown to be among the species vulnerable to 6PPD-quinone, the highly toxic compound formed when 6PPD from tires reacts with ozone in the air. Some 24 separate populations of Northwest coho, Chinook and steelhead have been declared at risk of extinction.
Meanwhile, other legal approaches also aim to eliminate the use of 6PPD in tires. Three Native American tribes are seeking to ban 6PPD under the federal Toxic Substances Control Act, which regulates the use of toxic chemicals in this country. In response to the tribes’ petition, the federal Environmental Protection Agency agreed to undertake a formal review to consider such a ban.
Also, in Oregon and California, the Center for Biological Diversity, an environmental group, has filed a notice threatening a lawsuit against state and federal highway agencies “for actions and inactions related to maintenance and management of roads affecting imperiled coho and Chinook salmon.” Road runoff that carries toxic tire chemicals into salmon streams is not covered by existing permits under the Endangered Species Act, according to attorneys in the case. They argue that the California Department of Transportation, the Oregon Division of the Federal Highway Administration and the U.S. Department of Transportation must immediately reconsider how highway construction and maintenance affects threatened and endangered species in light of new scientific findings about tire chemicals.
Starting in the 1950s, 6PPD has been infused in the rubber of tires to prevent degradation caused by ozone. Without the additive, tires are known to crack and degrade much more rapidly. For more than 20 years, salmon biologists have known that coho salmon were being killed by urban stormwater runoff, but the chemical cause remained a mystery until three years ago. That’s when environmental toxicologists were able to isolate 6PPD-Q and demonstrate that this single compound was responsible for the deaths of untold numbers of coho and other salmonids. [Editor’s note: Scientists affiliated with our publisher the University of Washington Puget Sound Institute discovered the compound in collaboration with researchers from Washington State University, NOAA and numerous other organizations.]
These graphics illustrate the harm caused by 6PPD-quinone for coho and other fish in some locations (above), and how best management practices for stormwater treatment can improve water quality, ecosystems, and human health (below). Illustrations: Washington State Department of Transportation.
While some toxicologists work to understand how the chemical affects fish, others are trying to find less-toxic compounds to protect tires from degradation. In Washington and California, programs established in state law are focused specifically on the search for less-toxic chemicals.
The U.S. Tire Manufacturers Association, which represents 85 percent of the industry in the U.S., supports the search for a safer tire additive. “Our members continue to research and develop alternative tire materials that ensure tire performance and do not compromise safety, consistent with our industry’s commitment to sustainability and respect for the environment,” according to a statement from Kim Klein, vice president of public affairs for USTMA.
Other research efforts are looking for ways to reduce the toxicity of road runoff before it spills into salmon streams. Studies have shown that filtering contaminated water through organic material, such as compost, can remove a large fraction of the tire-related compounds as well as other pollution. But installing such filtration on all roadways with existing technology poses an enormous challenge, not to mention cost, so all types of stormwater controls are being explored.
State agencies in Washington and California have undertaken formal processes that could require tire manufacturers to replace 6PPD in tires with one or more less-toxic chemicals. Although the laws in the two states are different, both require a finding that any replacement chemical must be safer than 6PPD-Q with strong considerations for effectiveness. Much is on the line, because tire degradation threatens to increase the rate of injury or death on the highways, officials say.
“Protection materials, such as 6PPD, are essential for tire performance and safety,” according to a statement from the tire manufacturers, “and any potential alternative must continue to ensure compliance with Federal Motor Vehicle Safety Standards (FMVSS) and other consumer, vehicle and tire manufacturer requirements. It is not a simple process to change the composition of tires; any change could affect the stopping distance of tires, durability, vehicle fuel economy, tire wear and other safety‐related components.
“While many potential alternatives are being considered, none has been demonstrated to be technically feasible for meeting federal safety requirements,” the statement continues. “Any premature prohibition on the use of 6PPD in tires would be detrimental to public safety and the national economy.”
In 2021, a few months after the discovery of 6PPD-Q, the Washington Legislature began funding a search for alternatives to 6PPD. The Washington Department of Ecology quickly published an initial list (PDF) of alternative chemicals but acknowledged that questions about their toxicity and effectiveness were not yet answered. Since then, more extensive evaluations of existing chemicals have been moving forward, even as scientists design new chemicals to meet demands for lower toxicity without compromising safety.
This past October, Ecology finalized assessment criteria to provide standards for comparing alternative chemicals to each other and to 6PPD. With a few additions, the criteria are identical to those used in the Safer Products for Washington program, which has now added 6PPD to its draft list of “priority chemicals.” These are considered the most dangerous chemicals in use today, allowing Ecology to undertake a formal step-by-step replacement effort.
Under the Safer Products law, approved by the Legislature in 2019, the Department of Ecology plays a central role in identifying safer chemicals for specific uses. Last May, the program completed its first round of review by issuing regulations to address toxic chemicals in 10 categories of consumer products, including flame retardants in electronic equipment and foam padding, bisphenol compounds in food and drink cans and in thermal-paper receipts, and ortho-phthalates in vinyl flooring and in personal-care products. (See also 2022 report to the Legislature.)
In a related but separate approach, Ecology and the Washington State Department of Health are developing a 6PPD chemical action plan, a strategy for addressing the problem on a holistic scale — from monitoring tire chemicals in the environment to finding feasible replacements. The plan, which will include recommendations to next year’s Legislature, is being developed with the help of experts from many agencies and organizations. Updates are provided on the 6PPD Action Plan website. Ecology also shares the latest information through a special 6PPD Subgroup within the agency’s broader Stormwater Work Group. Organizers compiled a list of questions with answers (PDF) from state agency officials following a June 2023 interagency presentation.
A somewhat different approach is being following in California, where the state’s Safer Consumer Products program places more of the assessment burden on manufacturers themselves. As of Oct. 1, any tire containing 6PPD is considered a “priority product,” as declared by the California Department of Toxic Substances Control, which issued a product-chemical profile (PDF) in March 2022.
By March 29 of this year, tire manufacturers must file a notice of intent to remove or replace the designated chemical or product, or else provide a preliminary report on alternatives, including information to compare various alternatives.
The U.S. Tire Manufacturers Association has supported California’s listing of 6PPD in tires but continues to stress the need for research to fill “data gaps.” The association has organized a consortium of 29 (originally 16) tire manufacturers, which is expected to complete the analysis by the March 29 deadline, according to Klein of USTMA. The association also is working with the U.S. Geological Survey to develop methods for evaluating potential alternatives to 6PPD in tires.
If the analysis concludes that no feasible alternative exists at the time of the report, the manufacturers may submit an abridged alternatives analysis report, according to Russ Edmondson, information officer for DTSC. California regulators could then require any number of actions, including chemical engineering to produce safer products, restrictions on chemical uses, management of tire disposal, information for consumers, and so on.
Meanwhile, a great deal of technical information about 6PPD and related issues, ranging from research to pilot projects, is being shared among a wide range of interested parties organized under the Interstate Technology and Regulatory Council, which established a Tire Anti-degradants (6PPD) Team in January 2023. That team issued a “focus sheet” (PDF) last summer describing the state of knowledge with respect to 6PPD and 6PPD-Q.
In addition to the Environmental Protection Agency’s regulatory role involving toxic chemicals, the agency has been funding grants for basic research, innovative stormwater projects and competition for alternative tire compounds. The EPA also announced this week that it has developed uniform analytical methods for detecting 6PPD-Q in water.
Under the Toxic Substances Control Act, the EPA has authority to require the testing and reporting of potentially harmful chemicals and to issue restrictions on the use of chemicals found to threaten the health of humans or the ecosystem. A petition from three tribes (PDF), submitted last August, calls on EPA to prohibit the manufacturing, processing and use of 6PPD in tires. The tribes are the Port Gamble S’Klallam and Puyallup tribes in Western Washington and the Yurok Tribe in Northern California.
Josh Carter, environmental scientist for the Port Gamble S’Klallam Tribe, issued a statement in August calling for immediate action. “To see 6PPD-Q kill the salmon that are reared in the Port Gamble S’Klallam Tribe’s own streams and from its own hatchery is an unconscionable slap in the face to a people who rely on salmon for their wellbeing, in addition to being a gross violation of the tribe’s rights as enshrined in the 1855 Treaty of Point No Point,” he said. “If EPA truly cares about protecting the environment and the tribe’s treaty rights, not just industry’s pocketbooks, it will act now.”
In November, EPA granted the tribes’ petition in a letter (PDF) and is expected to publish a “notice of proposed rulemaking” along with a call for relevant scientific data by this fall. Before taking formal action, EPA must evaluate the risk of injury or death from the chemical, considering both hazard and exposure. Rules can then be written to eliminate the chemical or otherwise reduce the risk. The EPA letter stresses that no commitments are being made to “specific rulemaking, timeframe or outcome.”
“EPA is responding to our tribal partners by taking action to protect the coho salmon, which are a key part of the tribes’ cultural identity and economic security,” Michal Freedhoff, assistant administrator for the EPA’s Office of Chemical Safety and Pollution Prevention, said in a written statement. “These salmon and other fish have suffered dramatic decreases in population over the years. Addressing 6PPD-quinone in the environment, and the use of its parent, 6PPD, is one way we can work to reverse this trend.”
In September, the Affiliated Tribes of Northwest Indians passed a resolution (PDF) at its annual convention in support of the petition. ATNI represents more than 50 member tribes from Washington, Oregon, Idaho, Montana, California, Nevada and Alaska.
Meanwhile, attorneys general from five states also joined the call for action by the EPA. The states are Washington, Oregon, Connecticut, Vermont and Rhode Island.
“Well over a billion dollars has been invested in habitat restoration in Washington,” states a joint letter (PDF) from the AGs, “and even more resources will be needed in the future. Through the efforts of federal, tribal, state and local governments, as well as NGOs, riparian areas have been restored, and in some cases dams have been or will be removed. Despite these efforts and expenditures, salmon runs are still declining in most of Washington’s rivers…
“Removing 6PPD and 6PPD-quinone from the environment would be of great benefit to fish populations, in particular coho salmon and steelhead,” the letter says. “The states strongly urge EPA to grant the tribes’ petition and act to protect the environment from this extremely harmful pollutant.”
The U.S. Tire Manufacturers Association urged EPA to deny the petition, because a suitable alternative has not been identified, but the association encouraged EPA to undertake a risk evaluation if warranted by the toxic chemicals law.
National Marine Fisheries Service
Another federal agency, the National Marine Fisheries Service, also called NOAA Fisheries, could be drawn into the discussion over 6PPD as a result of the lawsuit (PDF) filed in November against 13 of the country’s largest tire manufacturers. The lawsuit alleges violations of the Endangered Species Act and seeks to either ban the chemical outright or else force tire companies to acquire a federal “incidental take permit” spelling out mitigation measures to better protect vulnerable fish stocks.
An incidental take permit is an acknowledgment that some action — such as allowing toxic chemicals to get in the water — has an adverse effect on a listed species. Before obtaining a permit, the responsible party must develop a conservation plan that provides protections for the species at risk. The conservation plan must undergo public review and be approved by NMFS, and the applicant must demonstrate available financing to carry out the plan.
With fish, each defined population is considered a species. In Washington, Oregon and California, a total of 24 populations of coho, Chinook and steelhead are listed as endangered or threatened. Among coho, one population is listed as endangered and three are threatened; for Chinook, one is endangered and eight are threatened; and for steelhead, one is endangered and 10 are threatened. (An endangered species is a species or a population at high risk of extinction; a threatened species is one likely to become endangered in the foreseeable future.)
NMFS has already identified stormwater as a significant factor in the decline of all 24 species, according to the legal complaint filed by Earthjustice on behalf of two fishing groups. They are the Institute for Fisheries Resources, a nonprofit research and conservation group dedicated to protecting fisheries along the West Coast, and Pacific Coast Federation of Fishermen’s Associations, the largest trade organization of commercial fishing families on the West Coast.
“California’s Department of Toxic Substances Control (DTSC) has linked a 70 percent decline in coho in the San Francisco Bay Area between the 1960s and 1990s to the use of 6PPD in tires,” the complaint states. “DTSC has concluded that the presence of 6PPD-Q in California’s waterways continues to threaten the state’s remaining coho salmon populations and may jeopardize the recovery of this species.”
The U.S. Tire Manufacturers Association has chosen not to comment on the lawsuit as a matter of policy, said Klein, speaking for the organization. She added that she has no information about whether any company has approached NMFS about potential obligations under the law.
No hearings or briefings have been scheduled in the case.
Another possible ESA lawsuit
The Endangered Species Act is also the focus of a 60-day notice (PDF) of a possible lawsuit filed in June by the Center for Biological Diversity, an environmental organization. The suit alleges that federal permits issued for highway-maintenance projects are no longer adequate, since they were issued before the discovery that 6PPD-Q could be having a serious impact on listed salmon populations.
Previous evaluations examined the known impacts of highways, as required by the Endangered Species Act, but the law requires renewed consideration and consultation with NFMS when “new information reveals effects of the action that may affect listed species or critical habitat in a manner or to an extent not previously considered,” states the 60-day notice.
In a letter to 14 congressional representatives (PDF), NOAA Administrator Richard Spinrad said the National Marine Fisheries Service may need to address new findings about stormwater in multiple permits.
“To address the effects of 6PPD-quinone, federal action agencies will, in some cases, need to acknowledge and incorporate new and additional methods of stormwater management,” Spinrad wrote. “Such additional measures — for example, compensatory mitigation — might be necessary in areas where water quality impacts, caused by stormwater runoff, are threatening salmon recovery.”
Following the 60-day notice, the Center for Biological Diversity could have filed its proposed lawsuit as early as August, yet legal action is still awaiting a final decision, according to the group’s attorney Emily Jeffers. Some discussions have taken place with the involved agencies, she noted, and questions remain about whether state and federal highway officials are doing all they can to remove 6PPD-Q from stormwater using the latest system designs, also known as “best available technology.”
In addition to enforcing the Endangered Species Act, NOAA’s Northwest Fisheries Science Center is conducting research about the effects of 6PPD-Q on fish, as is the U.S. Geological Survey’s Western Fisheries Research Center, both in Seattle. NOAA also is involved in other projects, including the development of population models to estimate large-scale effects of the chemical and funding for experiments involving new stormwater systems.
Treatment systems and processes
Even if 6PPD is replaced in tires with a new chemical of lower toxicity, it will take years before the new tires are the only ones on the road and more years for the chemicals to work their way through the environment and out of the streams, experts agree. Finding new ways to reduce the toxicity of contaminated stormwater has become a major goal of federal, state and local governments along with research institutions.
Washington Department of Ecology and other agencies use the term “best management practices” to describe engineered treatment systems as well procedures and actions shown to be effective at reducing pollution. A review of existing techniques (PDF) with ideas for future research was published by Ecology in June 2022, soon after 6PPD was identified as a major problem in Washington’s waterways. It was followed that October with a report to the Legislature.
Sweeping streets with vacuum trucks, cleaning roadside ditches, and purging storm drains appear to be effective measures for removing dust particles and sediment that harbor 6PPD-Q and other toxic chemicals, according to the legislative report.
Downstream but before stormwater reaches natural waterways, significant amounts of pollutants, including 6PPD-Q, can be removed by diverting flows through artificial channels, called bioswales, according to reports. Allowing stormwater to soak into the ground also captures the chemical in the soil. Even more effective, it appears, is filtration through a mixture of organic materials, such as layers of compost along with sand. Yet filtering large volumes of stormwater, such as runoff from highways, presents a major challenge. Research is underway to test various systems in real-world applications.
Canadian researchers at the University of British Columbia reported promising results last year with a bioretention cell, sometimes called a rain garden, as reported in the journal Environmental Science and Technology. In a typical storm, more than 90 percent of the 6PPD-Q was removed by the single cell, according to the study.
Preliminary results also were reported as promising in a limited experiment (PDF) by researchers for King County and in a separate project by Long Live the Kings in partnership with the Nisqually Tribe.
Jen McIntyre, a toxicologist at Washington State University’s Puyallup Research and Extension Center, first revealed how such filtration, known as bioretention, could detoxify stormwater and save coho from certain death (Chemosphere, 2015). McIntyre’s current research, nearing completion, is exploring the effectiveness of different depths of bioretention media and how long they will last under various conditions.
Other WSU researchers recently published a new study about permeable pavements, a type of roadway surface designed to allow water to pass through rather than run off. The study demonstrated that more than 96 percent of the particles coming off tires can be captured within the pores of the pavement as stormwater penetrates. In tests of four types of pervious pavement, the amount of 6PPD-Q passing through was reduced by an average of 68 percent, according to findings published Jan. 15 in the journal Science of the Total Environment.
“Permeable pavements are a very promising type of green stormwater infrastructure, because they could treat this type of pollution where it’s generated, rather than downstream,” said author Chelsea Mitchell, who recently earned a PhD in environmental and natural resource sciences from WSU. The research project, described in a WSU news article, was led by Ani Jayakaran, a professor at WSU’s Puyallup Research and Extension Center.
The Washington Department of Ecology has been regulating stormwater for many years, first to control the rate of flow into streams and later to reduce sedimentation and pollution. Ecology officials are incorporating the latest information about 6PPD-Q into newly proposed stormwater regulations to protect streams from contaminated runoff. Depending on the outcome of ongoing research, additional requirements to control 6PPD-Q may be proposed in the future. Pilot projects designed before the discovery of 6PPD-Q are moving forward with additional monitoring to detect the tire chemicals. See Washington Department of Transportation Stormwater Retrofit (PDF) and Ecology’s Stormwater Action Monitoring.
Environmental justice and treaty rights
As researchers consider the effects of 6PPD-Q on human health, questions of environmental justice are beginning to focus on people living near highways, who may be more exposed to the toxic tire chemical than those living farther away.
The EPA has long recognized that highways pose increased health risks to nearby residents, because of excessive automobile pollutants released to the air, although actual exposures vary greatly from place to place.
“Pollution from the transportation sector has been a long-standing obstacle to advancing environmental justice, as many communities of color and low-income families live near areas where pollution from vehicles and engines is abundant and therefore experience disproportionate exposures to pollution,” according to a statement from the EPA.
When considering major government actions or policies, state and federal regulators must consider environmental justice, which the EPA defines as “the fair treatment and meaningful involvement of all people regardless of race, color, national origin, or income, with respect to the development, implementation and enforcement of environmental laws, regulations and policies.”
The chemical 6PPD-Q may be disproportionately exposing some populations because of where they live, their housing conditions or their cultural practices. Even though not much is known about human health effects from 6PPD-Q, possible routes of human exposure include breathing polluted air, eating exposed fish, drinking contaminated water, or contacting recycled rubber such as used on some playfields.
Native Americans tend to eat more fish and shellfish as a result of tradition and cultural practices. Some ethnic groups and commercial fishers are known to consume more fish than average, but exposure to 6PPD-Q depends on the type of fish, its environmental conditions and other factors. Health officials are near the beginning of an effort to determine a safe level of human exposure.
“People who already experience social stress from factors like poverty and racism are even more vulnerable when exposed to harmful chemicals,” according to an Ecology fact sheet (PDF) that explains why environmental justice is important when considering potential harm from toxic substances.
At the federal level, the concept of environmental justice has evolved through a series of executive orders, starting in 1994 with Executive Order 12898 signed by President Bill Clinton and continuing to 2023 with Executive Order 14096 signed by President Joe Biden. As outlined in these orders, environmental justice seeks to integrate a level of fairness in all government actions, reaching beyond the concept of anti-discrimination, as codified in the Civil Rights Act of 1964.
At the state level, a law passed in 2021, called Washington’s Healthy Environment for All — the HEAL Act — is the nation’s first state law providing for a coordinated approach to environmental justice, according to the Department of Ecology. The law requires Ecology and other agencies to identify and address environmental health disparities for overburdened communities and vulnerable populations, as defined in the law. Among the legal requirements, each agency must develop a community engagement plan and conduct environmental justice assessments for all significant actions.
With respect to 6PPD-Q, community engagement will be included in the development of a chemical action plan, which will propose strategies for addressing the chemical in ways that improve human health and the environment.
In setting priorities for highway-related stormwater projects, the Washington State Department of Transportation has included the goal of “addressing health disparities” along with three other major goals: salmon recovery, reduced pollution and cost effectiveness.
“The prioritization update (a map-based system) is being developed with input from tribes; federal, state and local governments; scientists; and others; and (it) includes consideration of tribal treaty rights and environmental justice principles, including the HEAL Act,” states the introduction to a draft of the web-based map, which is still undergoing expert review.
According to a legislative goal, 40 percent of grants and expenditures that create environmental benefits should be directed towards overburdened communities and vulnerable populations, according to Tony Bush, stormwater branch manager for WSDOT. “Our prioritization system for investments in stormwater retrofits is being developed with this HEAL Act requirement as a guiding factor,” he noted.
Improving stormwater infrastructure to reduce 6PPD-Q became a priority of the Washington Legislature in 2022, when lawmakers launched an initiative called “Move Ahead Washington” (PDF) and authorized $500 million for stormwater retrofits on highways over the next 16 years. The program places an emphasis on “green infrastructure,” the type that uses bioswales and natural filtration.
The prioritization map for highway-related stormwater projects incorporates a Health Disparities Map, developed by the Washington State Department of Health to identify communities subjected to environmental risk, also accounting for issues such as poverty and cardiovascular disease.
“Tribes can directly influence WSDOT’s stormwater retrofit prioritization and represent their interests by entering up to four top priority stormwater retrofit locations on WSDOT infrastructure,” Bush said, referring to access via an online survey. “When tribes enter in these locations, they are automatically bumped to the top of the list for consideration in WSDOT’s prioritization.”
Health disparities observed in Native American communities are often included with environmental justice considerations. But, beyond that, long-standing treaties guarantee tribal members the right to hunt and fish, providing a strong legal basis for sharing in decision-making, particularly decisions involving salmon and natural resources. Consequently, tribal members and tribal biologists are actively participating on many of the boards and committees dealing with the 6PPD-Q problem.
Guiding the government-to-government relationship between Washington state and 26 federally recognized tribes is a 1989 document called The Centennial Accord. The document establishes an agreed process for implementing major policies affecting tribes in Washington.
“Each Party to this accord respects the sovereignty of the other,” states the preamble to the accord. “The respective sovereignty of the state and each federally recognized tribe provide paramount authority for that party to exist and to govern. The parties share in their relationship particular respect for the values and culture represented by tribal governments.”
In 2012, the relationship was further cemented with a state law, RCW 43.376.
EPA, which has been asked by three Northwest tribes to ban 6PPD-Q under the Toxic Substances Control Act, also engages in “consultation” with the tribes, defined as a “two-way, government-to-government exchange of information and dialogue” between EPA and tribal officials. EPA’s Policy on Consultation with Indian Tribes was just updated this past December.
The petition to ban 6PPD in tires stresses that salmon and steelhead have been a “foundational part of tribal culture, religion and subsistence since time immemorial, and EPA must consider the environmental justice impacts of allowing continued harm to these culturally important species.”
Salmon also are important to the economy, supporting about 16,000 jobs in the commercial and recreational fishing industry, yet 29 percent of the historical populations have already been lost, according to the petition in support of EPA action. “The loss of these salmon and steelhead populations has already significantly diminished the ecosystems, cultures, and economies of the West Coast.”
Tanya Williams, Ecology’s 6PPD lead planner, said the HEAL Act is integral to the state’s response to the 6PPD problem. As scientists gain an understanding about the effects of 6PPD-Q, policymakers in search of solutions will prioritize the needs of communities and populations disproportionately impacted by the harms caused by the chemical, she said.
The stunning discovery of 6PPD-Q as a highly toxic chemical launched dozens of research projects around the world, she noted, but concerns about a chemical connected with everyone’s vehicles have yet to penetrate deeply into the minds of most people. That could change, she added, as more scientific studies better describe the risks to various species, including humans.