Authorities in the Chilean capital of Santiago have declared an environmental emergency due to the extremely high levels of pollution in the South American metropolis.

As a result of the emergency – the first in the country since 1999 – 900 industries have been forced to shut up shop and 1.7 million cars have been ordered off the road for a 24-hour period, which could be stretched even further if the authorities deem fit.

However, it is unclear which industries have been temporarily closed, according to Reuters.

Additionally, the 6.7 million strong population of Santiago have been advised against any outdoor exercise although this is not mandatory and could prove to be problematic as the country is currently hosting South America's pinnacle football tournament, the Copa America.

The Chilean Environment Ministry released a statement saying: "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.

"We call for people to respect the measures in order to help reduce the pollution of Santiago."

The ministry explained that it issued the warning due to the city's lack of rain and winds in recent times, which has increased the amount of PM2.5 – tiny breathable particulate matter 2.5micrometres in diameter.

According to United States Environmental Protection Agency's website: "'Particulate matter,' also known as particle pollution or PM, is a complex mixture of extremely small particles and liquid droplets. Particle pollution is made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles."

Particulate matter can be harmful as it can enter the lungs and bloodstream, which can cause difficulty breathing and has been linked to heart disease. It is also harmful to the environment.