Smoke from forest fires smothering Moscow adds to health problems of “brown clouds” from Asia to the Amazon and Russian soot may stoke global warming by hastening a thaw of Arctic ice, environmental experts say.

“Health effects of such clouds are huge,” said Veerabhadran Ramanathan, chair of a U.N. Environment Programme (UNEP) study of “brown clouds” blamed for dimming sunlight in cities such as Beijing or New Delhi and hitting crop growth in Asia.

The clouds — a haze of pollution from cars or coal-fired power plants, forest fires and wood and other materials burned for cooking and heating — are near-permanent and blamed for causing chronic respiratory and heart diseases.