Children, elderly and sick people are most vulnerable to harm

Liu Chuanhe, a doctor in the respiratory department of the Capital Institute of Pediatrics, said he has received more patients in the past several days.

"There are indeed more patients, including some whose syndromes have worsened, during smoggy days," Liu said.

Liu said he examined more than 50 children on Tuesday, most of them suffering from respiratory conditions such as asthma.