Kathmandu, April 9 -- Every evening, as she walks home from her German language class in Putalisadak, Sneha Bhattarai carries biscuits in her bag. Her route through Thapathali, past Norvic Hospital and down the Maternity Hospital road to Sankhamul, is familiar, not just to her, but to the 8 to 12 street dogs who await her small treats.

But last Friday, something felt different. The dogs weren't the same.

"They didn't run to me like usual," said Bhattarai. "One of my favourite dogs near Rosebud School was quite aggressive, coughing, sneezing and avoiding the biscuit. It looked like it was struggling to breathe."

Bhattarai, wearing a mask herself, then thought, it must be the pollution. And as experts would later confirm, her hunch was r...