Smog has reached alarming levels in Beijing and much of northern China, forcing parents to keep their children inside and sending many to the hospital.

Monitors say that air pollution has soared past healthy levels, reaching up to 40 times the standard set by the World Health Organization in some parts of the country, according to official readings reported in the Associated Press.

Hospitals reported increases of up to 30% in the number of patients reporting breathing problems, and Chinese officials have warned that bad visibility and hazardous air conditions will likely last until Wednesday, the Guardian reported.