1. **Particulate Matter (PM)**:

   - **PM10**: Particles with a diameter of 10 micrometers or smaller. These can penetrate the respiratory system and cause health issues.

   - **PM2.5**: Fine particles with a diameter of 2.5 micrometers or smaller. They can enter the bloodstream and are associated with serious health problems, including heart and lung diseases.

2. **Nitrogen Dioxide (NO2)**:

   - A reddish-brown gas that is a byproduct of combustion processes, particularly from vehicles and power plants. It can irritate the respiratory system and contribute to the formation of smog.

3. **Sulfur Dioxide (SO2)**:

   - A colorless gas with a sharp smell, primarily produced from burning fossil fuels (like coal and oil) and industrial processes. It can cause respiratory problems and contribute to acid rain.

4. **Carbon Monoxide (CO)**:

   - A colorless, odorless gas produced by incomplete combustion of fossil fuels. High levels can be harmful, particularly in enclosed spaces, as it can interfere with the blood's ability to carry oxygen.

5. **Ozone (O3)**:

   - While ozone in the upper atmosphere protects against UV radiation, ground-level ozone is a harmful pollutant formed by the reaction of sunlight with pollutants like volatile organic compounds (VOCs) and nitrogen oxides (NOx). It can cause respiratory issues and other health problems.

6. **Volatile Organic Compounds (VOCs)**:

   - A group of organic chemicals that can evaporate into the air. Common sources include vehicle emissions, industrial processes, and household products (like paints and cleaners). Some VOCs can contribute to the formation of ground-level ozone and smog.

7. **Lead**:

   - Although its levels have significantly decreased due to regulations, lead can still be found in urban air, primarily from industrial sources and past use of leaded gasoline. It can have serious health effects, particularly in children.

8. **Ammonia (NH3)**:

   - Emitted from agricultural activities, waste treatment plants, and industrial processes. While it is less harmful in itself, it can react with other pollutants to form particulate matter.

## Conclusion

Urban air is often a complex mixture of these pollutants, which can vary in concentration depending on factors such as traffic, industrial activity, weather conditions, and geographic location. Monitoring and managing these pollutants is crucial for protecting public health and improving air quality in urban areas.