Scientists classify outdoor air pollutants into two categories. Primary pollutants are harmful chemicals emitted directly into the air from natural processes and human activities (Figure 18-4, center, p. 472). While in the atmosphere, some primary pollutants react with one another and with the basic components of air to form new harmful chemicals, called secondary pollutants (Figure 18-4, right). With their high concentrations of cars and factories, urban areas normally have higher outdoor air pollution levels than rural areas have. But prevailing winds can spread long-lived primary and secondary air pollutants from urban and industrial areas to the countryside and to other urban areas. Good news. Over the past 30 years, the quality of outdoor air in most developed countries has improved greatly. This occurred mostly because grassroots pressure from citizens caused governments to pass and enforce air pollution control laws. Bad news. According to the World Health Organization (WHO), more than 1.1 billion people (one of every six people on the earth) live in urban areas where outdoor air is unhealthy to breathe. Most of them live in densely populated cities in developing countries where air pollution control laws do not exist or are poorly enforced. However, the biggest pollution threat to poor