According to two new World Health Organization reports, about 1.7 million children under the age of 5 die each year because of environmental hazards. It's the first such estimate of the child death toll from environmental causes."That terrible figure" makes up about a quarter of child deaths under 5, says Dr. Maria Neira, WHO's public health and environment department director and lead author on the reports. In addition, children can experience mental and physical developmental disorders and an increased lifelong risk for certain diseases because of exposure to pollutants.The causes of death include respiratory diseases and illnesses spread by water pollution like diarrhea and intestinal infections, as well as non-communicable diseases like cancer, congenital diseases and asthma. These might arise from air pollution or from exposure to toxins in the environment like heavy metals.Because unhealthy environments might include stagnant water where mosquitoes breed, deaths from mosquito-borne diseases like malaria or dengue are included. "WHO is looking for things they can prevent so they define environment broadly," says Dr. Philip Landrigan, the dean for global health at the Icahn School of Medicine at Mt. Sinai, who did not work on the report. "It does not in any way detract or diminish the gravity or credibility of the findings," he says.The report finds that indoor and outdoor air pollution is one of the most extreme hazards to children's health — and it's on the rise. Even though air pollution levels have dropped over the years in high-income countries, on average the global figures for air pollutants have risen 8 percent between 2008 and 2013, according to the WHO. Air pollution is associated with a majority of the 600,000 child deaths from respiratory illnesses each year. Air pollution is the worst in urban areas with heavy traffic or coal-fired plants, and in homes that still use organic fuels like wood or animal dung for cooking. And pollution can take a toll in later years as well, she says: "The earlier you are exposed to air pollution as a child, the risk [increases for] chronic respiratory diseases like asthma, lung cancer or stroke."Electronic waste has also become a rising source of worry, Neira says. In many low- and middle-income countries, electronic parts are illegally or informally recycled. "Sometimes for $2 a day, a mother will be trying to recycle heavy metals in a computer or old televisions or mobile phones. And children, because they have very little fingers, will help as well." WHO has found that levels of heavy metals in the blood of young children engaged in these activities can sometimes exceed 30 times the recommended level, Neira says.The report helps to clearly establish the link between pollution and children's health, Landrigan says. "It nails down the connectionand documents what many of us have known and been saying for years — that unhealthy environments make people unhealthy," he says. "That's very relevant at this time. People in Washington are talking about relaxing regulations, and they're not acknowledging — maybe they don't know — allowing waterways and the air to become polluted will result in death."These deaths are also preventable, Neira says. Policies and regulations that improve housing, sanitation, clean water and emissions often result in large benefits to children's health, she says. "I want those figures [in the report] to provoke more action. To make sure that policymakers will not have the excuse of saying 'We didn't know.' "Freelance writer Angus Chen is on Twitter @ angRchen Copyright 2017 NPR. To see more, visit http://www.npr.org/.