More than 600,000 children died from breathing polluted air in 2016, according to a World Health Organization report published Monday.

The agency estimated that 93 percent of children under the age of 15 across the world are breathing in dirty air that puts their health and development at risk.

Children are affected before they are even born, the report found. When pregnant women are exposed to polluted air, they are more likely to have premature births and have babies that are born underweight. Officials said exposure while in the womb was leading to damage in brain development, resulting poorer testing in schools and impeding motor functions.

"This is not only new but terribly shocking," Dr. Maria Neira, WHO's director of environmental and social determinants of health, said of the pre-birth findings.

The report was launched as WHO prepared to hold its first Global Conference on Air Pollution and Health, set to begin Tuesday. Dr. Tedros Adhanom Ghebreyesus, the WHO’s director general, last week called air pollution the "new tobacco" in terms of public health priorities.

Officials estimated in the report out Monday that 1 in 10 deaths among children under age 5 are caused by air pollution. Children are particularly vulnerable to illnesses from toxic air because they breathe faster than adults. Pollution can trigger asthma, childhood cancer, and can lead to heart disease later in life.

Children around the world aren't just exposed to dirty air outside, but inside their own homes, where families burn wood and kerosene for cooking, heating, and lighting.

Officials on the call with reporters said it was important for countries to push for renewable energy and to reduce the use of coal and gas. They said the U.S. had a low level of air pollution relative to other countries, attributing the progress to the Clean Air Act, but cautioned that actions to roll back progress could worsen health.

There also are clusters of poor air quality in every country, warned Dr. Sophie Gumy, a scientist in WHO's department of environmental and social determinants of health.

The Trump administration has praised the progress the U.S. has made in achieving cleaner air, but is also rolling back regulations from the Obama administration that aimed to curb pollution from power plans, cars, and oil wells.