Ambient air quality, however, is important to consider when examining human health and the environment. Even though the data are not comprehensive, they are important to include as one measure of comparing relative environmental health threats among countries. The leaded gas emission level obtained from multiplying these variables was then weighted with the urban population density to arrive at the final lead exposure measure. The market share of leaded gasoline and the maximum lead concentration of the gasoline are rapidly changing in many countries of the world. Generally, countries are moving away from using leaded gasoline with high concentrations of lead to either gasoline with lower concentrations of lead or to all lead-free gasoline.
Exceptions do occur, however.
To estimate population density of urban areas, WRI combined a database that determines the builtup areas in countries from satellite images of city lights at night with data on urban population from the U.N. Population Division. This database contains the locations of stable lights, including frequently observed light sources such as gas flares at oil drilling sites. Time series analysis is used to exclude temporary light sources such as fires and lightning. Although the lights database provides the best available source for data on builtup areas for countries of the world, it may underestimate the builtup area for countries with densely populated but low-lit areas, such as squatter settlements. However, WRI used the database because it is the best source of information for calculating a relative estimate of urban populations potentially exposed to leaded gas emissions.