Pollution claimed the lives of nine million people in 2015, one in every six deaths that year, according to a study published in The Lancet medical journal on Friday.
Almost all the deaths, 92 percent, happened in low- and middle-income countries, researchers said, with air pollution the main culprit.
Almost half of the total toll came from just two countries -- India and China – they said.
In rapidly-industrialising countries such as India, Pakistan, China, Bangladesh, Madagascar and Kenya, pollution can account for as many as one in four deaths, they added.
"Pollution and related diseases most often affect the world's poor and powerless, and victims are often the vulnerable and the voiceless," said co-author Karti Sandilya of Pure Earth, an anti-pollution NGO.
"As a result, pollution threatens fundamental human rights, such as the right to life, health, wellbeing, safe work, as well as protections of children and the most vulnerable."
With global welfare losses of about $4.6 trillion (3.9 trillion euros) per year, the economic cost of pollution-related deaths and disease is also concentrated in the developing world.
"Proportionally, low-income countries pay 8.3 percent of their gross national income to pollution-related death and disease, while high-income countries pay 4.5 percent," said the researchers.
Aside from outright poisoning, pollution causes an array of deadly ailments such as heart disease, stroke, lung cancer and chronic obstructive pulmonary disease.
The deadliest form, responsible for more than two-thirds of deaths, was air pollution, they added.
In a separate comment, The Lancet editors Pamela Das and Richard Horton said the report came at a "worrisome time, when the US government's Environmental Protection Agency, headed by Scott Pruitt, is undermining established environmental regulations."
The latest findings, they added, should serve as a "call to action".