Santiago regional governor Claudio Orrego said the emergency measures, to come into force Monday, would require 3,000 factories and other businesses to shut down. The move will also force 40 percent of the capital's 1.7 million cars to stay off the roads.

The emergency measures are the first to be implemented since 1999. The emergency status is the highest on the alert scale, and only applies when the air quality index reaches hazardous levels.

"We're currently facing unusual conditions, with one of the driest Junes in over 40 years as well as really bad air circulation conditions in the Santiago valley in recent days, which boosts the concentration of contamination," the Environment Ministry said in a statement.

The restrictions are expected to be in place for 24 hours, but can be extended if there are no improvement in conditions.

Residents urged to avoid outdoor exercise

It's not unusual for the city of 6.7 million to be shrouded in smog, especially in the winter months when an increased dependence on wood-burning heaters significantly worsens air quality.

Santiago lies in a bowl-shaped valley flanked by mountains, and concentrations of tiny particulate matter known as PM2.5 can build up during long periods without rain and wind. Particulate pollution is small enough that it can lodge deep in the lungs and enter the bloodstream, and it has been linked to a number of health problems, including heart disease and breathing difficulties.

Authorities have urged Santiago residents to avoid outdoor exercise while the emergency measures are in place. Chile is currently hosting the Copa America football tournament, and a number of the matches are scheduled to be held in the capital. However, the regional soccer federation does not allow matches to be cancelled due to air quality.

nm/cmk (Reuters, AFP)