LAHORE, Pakistan (AP) — Dangerously poor air quality forced Pakistani authorities in the cultural capital of Lahore on Monday to close primary schools for a week, government officials said.

The measures were part of a larger effort to protect children from respiratory and other illnesses in the city of 14 million people.

Toxic gray smog has sickened tens of thousands of people, mainly children and the elderly, since last month, when air quality began to worsen in Lahore, the capital of eastern Punjab province, bordering India.

The government has also banned construction work in certain areas and fined owners of vehicles that emit smoke. Schools will remain closed for a week due to pollution, according to a government notification.

The concentration of PM 2.5, or tiny particles, in the air was approaching 450, considered dangerous, the Punjab Environmental Protection Department said.

Lahore was once known as a city of gardens, which were omnipresent during the Mughal era from the 16th to the 19th century. But rapid urbanization and increasing population growth have left little room for greenery.