Until a few decades ago, the popular but falsely reassuring belief was that babies in the womb were perfectly protected by the placenta and that children were just “little adults,” requiring no special protections from environmental threats. We now know that a host of chemicals, pollutants and viruses readily travel across the placenta from mother to fetus, pre-polluting or pre-infecting a baby even before birth.

Toxic chemicals like lead, certain air pollutants, pesticides, synthetic chemicals and infectious agents like Zika can derail the intricate molecular processes involved in a fetus’s healthy brain development. So can physical and social stress experienced by the mother.

At a time when we should be spending more on research and prevention of those threats, President Trump would do the opposite. He would cut the budget of the Environmental Protection Agency by 31 percent, including cuts to scientific work on chemical safety. He would slash money for biomedical research and programs to fight outbreaks of infectious disease. We need more spending in those areas, not less. We need more testing of chemicals before they are marketed, not less.

Toxic exposures are shockingly prevalent. Analysis of biomonitoring data from the Centers for Disease Control and Prevention finds dozens of toxic chemicals, pollutants and metals in pregnant women, many of which are also found in cord blood of newborns. These include pesticides sprayed in inner-city buildings and on crops, flame retardants used in furniture, combustion-related air pollutants from fossil-fuel-burning power plants and vehicles, lead, mercury and plasticizers. All have been shown in epidemiologic studies in the United States and elsewhere to be capable of damaging developing brains, especially while babies are exposed in utero or in their early life.

This is why it was particularly distressing that the new head of the E.P.A., Scott Pruitt, recently rejected the scientific conclusion of the agency’s own experts, who had recommended banning one of the nation’s most widely used insecticides, chlorpyrifos. The experts made their judgment on the basis of many years of research indicating that chlorpyrifos was linked to significant harm to children, including diminished cognitive ability.

Research has also conclusively shown that climate change, caused in large part by carbon dioxide emitted by burning coal and other fossil fuels, is linked to more heat-related disease, malnutrition, infectious disease, trauma and mental health problems from extreme natural disasters like flooding. Those consequences can directly or indirectly affect early brain development, the cognitive and behavioral functioning of children and their ability to learn.

And yet Mr. Trump, as of Wednesday, was considering pulling the United States out of the Paris Climate Accord and also wants to abandon his predecessor’s Clean Power Plan to cut carbon emissions from power plants.

So uniquely vulnerable are the young that the World Health Organization estimates that children younger than 5 bear more than 40 percent of the global burden of disease caused by environmental risk factors and 88 percent of the disease burden caused by climate change. A notable increase in developmental problems in children worldwide has paralleled the proliferation of synthetic chemicals in our air, water, food and consumer products and the mounting impacts of climate change. About one in six children in the United States is affected by a developmental disability. These are complex disorders with multiple causes — genetic, social and environmental — often interacting with one another to increase risk.

All the world’s children are potentially exposed and at risk, at a great price to society. The estimated medical and/or economic costs of I.Q. loss and behavioral disorders attributable to just a few environmental toxicants indicate the enormous benefits of prevention: approximately $56 billion in 2008 for lead poisoning and prenatal mercury exposure in the United States; 146 billion euros (about $164 billion) each year attributed to prenatal organophosphate pesticide exposure in the European Union. The economic costs of toxic air pollutants and climate change from combustion of coal, oil and other fossil fuels amount to many billions of dollars a year in the United States.

Worldwide, according to the W.H.O., about three million deaths a year are linked to ambient air pollution; by 2030 the global health cost of a few climate-related diseases that disproportionately affect children (diarrhea, malnutrition, malaria and heat stress) will be as high as $4 billion a year.

Such tragic and costly consequences are preventable. The benefits of policies to reduce toxic exposures have been clearly demonstrated. Lead levels in children’s blood dropped in the late 1970s following legislation aimed at reducing or eliminating lead in house paint and gasoline. Concentrations of a neurotoxic residential-use pesticide dropped sharply in cord blood after the E.P.A. prohibited its use in 2001. In California, levels of various toxic flame retardants in the blood of pregnant woman and in breast milk fell since the state ban on those chemicals took effect in 2006. And in New York City, we saw a decline in harmful pollutants measured in air samples from personal monitors worn by pregnant women as clean air policies were enacted beginning two decades ago.

But as we saw with the tragic lead poisonings of children in Flint, Mich., much more needs to be done.

The problem is deep and systemic, resulting from lack of adequate government regulation requiring testing of chemicals before they are marketed, and from the failure to take prompt action once there is scientific evidence of harm. The E.P.A. must be able to act promptly to eliminate known brain-damaging chemicals and ensure that new chemicals or chemicals proposed for a new use undergo thorough testing before manufacturing.

With respect to climate change, federal policies and rules that promote clean energy, restrict climate-altering emissions from power plants, vehicles, industrial processes, and natural gas production and support the Paris Climate Accord are essential. They must not be weakened.

Our children’s health and future depend on it.