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 broader effort to protect children from respiratory and other illnesses in this city of 14 million.

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

The government also banned construction work in certain areas and imposed fines on owners of smoke-emitting vehicles. 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 approached 450, which is considered dangerous, the Punjab Environmental Protection Department said.

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