The air quality certificate, also known as a Crit’air sticker, is a secure sticker to be affixed to the windscreen. It indicates the vehicle’s environmental class, based on pollutant emissions into the atmosphere. The least polluting vehicles are thus identified and prioritized during peak pollution periods. Vehicles are divided into six environmental classes, each color-coded from green to gray, or from zero (green sticker) to five (gray sticker). The most polluting vehicles are not classified, and therefore do not qualify for the air quality certificate. These stickers are thus used to grant access (or not) to the LEZ.

The Crit’Air sticker classifies vehicles according to their pollutant emissions. The aim is not to tax the most polluting vehicles, but to restrict them from access to certain areas such as city centers or during peak pollution periods.