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Amid a summer when wildfire smoke has often filled the skies, a new study is adding to the body of research about the growing impact of fire-fueled air pollution, this time raising the alarm about prenatal health.

Wildfire pollution has become an increasing proportion of the pollution that pregnant mothers and their unborn babies are exposed to, according to a study published this month.

The new study by University of Maryland researchers found that out of the air pollution unborn babies are exposed to, the share of exposure to wildfire-generated microscopic air pollution more than doubled between 2003 and 2019.

The study did not discuss specific health outcomes for the prenatal exposure and births it analyzed, but previous studies have suggested that fine-particulate-matter pollution, known as PM 2.5, caused by wildfires can be more harmful than other forms of air pollution.

Studies have linked wildfire smoke to poor pregnancy outcomes and a variety of early-childhood health issues. The particles are small enough to enter the lungs and bloodstream, and they have also been linked to lung damage, respiratory disease, and cancers.

“We know that if we prevent PM 2.5 exposure in the prenatal period, these infants are going to grow to be healthier, more productive adults,” said Michel Boudreaux, an associate professor of health policy and management at the University of Maryland and one of the study’s authors.

The research also refers to the continued way pollution from wildfires cuts into improvements made in the United States’ air quality in the past two decades.

Researchers credit the Clean Air Act for reducing pollution from cars and industrial facilities.

But wildfire smoke has cut into those gains, according to several studies in recent years. A 2023 study found that, since 2016, wildfire smoke has eroded the equivalent of around four years of air quality progress in most of the country.

Wildfire pollution occurs inconsistently, Boudreaux said, flaring up when mega-fires break out in different regions.

It remains a small share of the PM 2.5 Americans are exposed to, but annual levels of wildfire smoke stayed consistent as other sources of pollution declined over the past two decades, the new study found.

Boudreaux’s study modeled air pollution across the country from 2003 to 2019, both including and excluding pollution created by wildfires, and used birth records to estimate the air pollution exposure to prenatal infants over this period. Researchers also analyzed the regions they studied by demographics and access to healthcare services.

The study found that the proportion of wildfire smoke in annual air pollution has increased by several percentage points from 2003 to 2019.

The greatest increase was in the Northwest, where around 13% of prenatal PM 2.5 exposure could be attributed to wildfires by the end of the decade. Regions with poor access to hospital facilities and maternal care, which were largely rural, had lower average PM 2.5 levels but faced the largest proportion of pollution from wildfires.

The new study also found that wildfires accounted for more than 70% of “exceedance days” — days where PM 2.5 levels exceeded 35 micrograms per cubic meter, an Environmental Protection Agency standard — by 2017 and 2018. Wildfire-attributable exceedance days stayed constant over the period studied, while exceedance days from other sources declined sharply.

“Because we’ve done such a good job of decreasing these non-wildfire sources, the share of ‘exceedance days’ that are due to wildfire smoke has increased,” Boudreaux said. “If we want to continue making progress on this … we have to start thinking about, how do we protect people from wildfire smoke?”

Academics not involved in the study said the work was limited by its scope and that the 2020s have seen a larger increase in wildfires after the study’s time frame. The five-year period from 2020 to 2025 was the smokiest on record in the United States, a study last year found.

“The last six years have just looked fundamentally different,” said Marshall Burke, a global environmental policy professor at Stanford University.

Michael Brauer, a professor at the School of Population and Public Health at the University of British Columbia, said further research is needed on the health risks of wildfire smoke, such as the effect of spikes in wildfire pollution at different stages of a pregnancy.

Burke said the study’s focus on prenatal exposure highlighted the breadth of air pollution’s impact across the population.

“We typically think of vulnerability to air pollution in terms of people with respiratory or other preexisting conditions,” Burke said. “But here, this is showing that pregnancy … really widens the lens of who we think as vulnerable.”

President Donald Trump issued last week a sweeping executive order aimed at changing how and when American children are vaccinated, defying guidance from major medical groups and sparking confusion among parents.

The directive calls for reducing the number of recommended childhood vaccines, spacing out the shots, and splitting up the MMR (measles, mumps and rubella) vaccine. The White House says the effort is to align the United States with its peer nations. The potential changes would diverge from long-standing federal guidance around immunizations, which medical experts credit with halting the spread of deadly preventable diseases in the United States.

But the bottom line is that federal vaccine recommendations have not changed yet.

Implementing Trump’s directives would require a lengthy administrative process and probably a legal battle, even as Trump has privately urged his deputies to move faster.

“An executive order is not an automatic change to the schedule, because that’s not the legal way it’s done,” said Dorit Reiss, a professor at the University of California College of the Law at San Francisco whose research focuses on public health law.

States, not the federal government, generally set vaccination requirements for school attendance.

The order has drawn opposition from major medical organizations, such as the American Medical Association, the American Academy of Pediatrics, the American Cancer Society Cancer Action Network, and the American College of Obstetricians and Gynecologists.

In January, federal health officials reduced the number of vaccines recommended for children, but a federal judge halted those changes in March, citing a flawed process.

Has the childhood vaccine schedule changed since Trump’s executive order?

No, there have been no changes to the childhood immunization schedule.

Executive orders are documents guiding federal agencies to pursue policies important to the president.

The White House is calling for the Department of Health and Human Services’ Task Force on Safer Childhood Vaccines to craft proposed changes within 90 days. The long-defunct panel was revived last year after a push from anti-vaccine activists. That task force makes recommendations to improve vaccine safety, not to recommend what should be the optimal vaccine schedule, Reiss said.

Legal experts say that’s not how vaccine changes are typically made. Usually, vaccine recommendations come from an independent vaccine advisory panel to the Centers for Disease Control and Prevention. The agency then decides whether to sign off on recommendations from the Advisory Committee on Immunization Practices.

“The task force isn’t the entity Congress designated to make vaccine recommendations,” said Richard H. Hughes IV, an adjunct professor of law at Georgetown and attorney at Epstein, Becker & Green who also represents plaintiffs suing the administration over previous vaccine changes.

What should I do about getting my children ready for school?

Medical experts say parents are likely to be confused by the latest announcement. They should talk to their pediatricians.

The immunization schedule from the American Academy of Pediatrics, endorsed by major medical organizations, has not changed.

The timing of shots, and how they are grouped, has been well studied so they are given when the immune system is best able to be trained to recognize and make the body stronger to fight off an infection from a particular disease if it is exposed to one, said Andrew Racine, president of the AAP and a pediatrician who has been treating children for more than 40 years.

“When parents have questions about vaccines or about any other aspect of their children’s health, we encourage them to speak with their pediatricians because pediatricians are best positioned to give parents sound advice about vaccines or anything else,” he said.

Will insurers cover the shots?

Insurers probably would still cover childhood vaccines even if Trump administration officials change the schedule.

“If this EO were to withstand legal scrutiny, it appears insurers should still cover recommended vaccines for free,” Jennifer Kates, a senior vice president at KFF, a health policy research and news organization, wrote in an email. She added that spacing the shots over multiple visits is already an option for parents.

Major insurers have also previously indicated they would continue to cover vaccines even in the face of potential changes. White House officials told reporters on Monday that they believed health insurers would continue to cover vaccines currently on the childhood immunization schedule regardless of whether recommendations change.

What changes does Trump want?

Taken together, Trump’s proposed changes would mark his most significant attempt to fundamentally alter vaccine policy in the United States.

Here are some of the order’s main directives:

• Fewer universally recommended shots: Vaccines against 11 diseases — including measles, mumps, rubella, polio and whooping cough — would remain recommended for all children. Immunizations for other diseases, such as hepatitis A and B, would be recommended for certain high-risk groups or through shared decision-making between parents and doctors. Both the flu and the coronavirus shots would be included in the shared decision-making category.
• Break up the MMR shot: Trump wants the combined measles, mumps, and rubella vaccine replaced with three individual shots, which are not currently available in the United States. Such a shift would diverge from the practices of several peer nations federal health officials have cited as models for change.
• Alternative adjuvants to aluminum: The order seeks alternatives to aluminum-containing ingredients used to strengthen the body’s immune response to some vaccines. Some high-level Food and Drug Administration officials believed last year that it would be infeasible to take aluminum adjuvants out of vaccines.
• Space out immunizations: The order states that “to the maximum extent feasible, all childhood immunizations should be administered at separate medical visits.”

Should I be getting my child individual shots of the MMR vaccine?

Individual shots are not available in the U.S., so practically speaking, that is not an option.

And making them available would not be as simple as splitting the existing MMR vaccine into three shots. Merck, which makes one of the MMR vaccines used in the U.S., said each individual vaccine would essentially be considered a new investigational product and would require development, clinical trials, and FDA review to demonstrate its safety and effectiveness.

The process could take years — potentially as many as 10 — before the vaccines could be approved and manufacturing and commercialization could begin, a Merck spokesperson said in a statement Tuesday. White House officials said Monday they plan to work with the private sector to make that option available.

Splitting up the MMR vaccine would also differ from the approach taken by several comparable countries the Trump administration has cited as models for U.S. vaccine policy. Denmark, Australia, and Canada give MMR immunizations in a combination vaccine. Britain this year began replacing the MMR shot with a four-in-one vaccine that also protects against chicken pox.

Major medical organizations strongly recommend the combination MMR vaccine, the safety and effectiveness of which has been well studied.

Two doses of the vaccine are 97% effective in preventing measles, which can result in serious complications, including pneumonia, swelling of the brain, and, in rare cases, death from respiratory and neurological complications.

Medical experts and public health officials have already seen a drop in the percent of parents willing to give the MMR vaccine to their children, and as a consequence, U.S. measles cases are at a 35-year high.

Merck said there is no published scientific evidence showing a benefit from separating the MMR vaccine into three individual shots. Combination vaccines can also make it easier for children to receive all their recommended vaccines on schedule, the company said; giving the components individually would require more injections and could lead to delayed or missed immunizations.

Why is this happening now?

Trump has long questioned whether the number of childhood vaccines can cause autism, in defiance of decades of research finding no link. The president’s musings have become more prominent during his second term with the selection of Robert F. Kennedy Jr. to lead the nation’s health department. Kennedy, the founder of a prominent anti-vaccine group, has a lengthy history of disparaging vaccines and has sought to reshape the nation’s immunization system, to the alarm of public health experts who say he is eroding confidence in long-standing measures.

Medical and public health groups say there is no new scientific evidence that would justify the changes Trump is seeking. Instead, they say the executive order comes amid a broader push by the administration to overhaul federal vaccine policy.

“There’s nothing that has changed. The science about vaccines and their efficacy has not changed,” the AAP’s Racine said. “The distribution of viruses and other pathogens in our environment has not changed. The children in this country have not changed in the last 48 hours.”