Fossil fuels are the most common resource used to produce energy in Missouri, accounting for approximately 80% of the total energy portfolio. Fossil fuels like coal, natural gas and oil and gas are key to the affordability of Missouri energy. However, when burned for energy, fossil fuels emit pollutants in the air, like mercury, lead, carbon dioxide, sulfur oxide and nitrous oxide. These emissions are toxic to humans and wildlife that breathe them. Health effects in humans can range from asthma and breathing difficulties, to brain damage, heart problems, cancer, neurological disorders and premature death. Missouri has had and continues to experience nonattainment in different areas across Missouri. These areas represent locations where criteria pollutants exceed federal limits. Criteria pollutants are air pollutants with national air quality standards that define the allowable amount of these substances in the air and have been shown to have direct effects on health.