Air quality index (AQI) is a metric developed to communicate levels of pollution over a given location. In a sense they can be looked at similarly to a weather report, as they inform an individual about the conditions of their environment. Different countries have their own indices which correspond to differing air quality standards. This means that a "moderate" level of air pollution in one place might be considered "low-risk" in another.
For the indices of different countries, visit the links below: