The six main pollutants monitored by the EPA that can have serious health consequences are particulate matter, ground-level ozone, lead, carbon monoxide, sulfur dioxide and nitrogen dioxide.[8] Particulate matter consists of a mix of solid and liquid matter such as dust, dirt, or smoke that makes up two categories, PM10 and PM2.5, the former being anything smaller than 10 micrometers and the latter being anything smaller than 2.5 micrometers.[8] PM10 can get deep within your lungs while PM2.5 is so small it can get into your blood stream both of which cause damage and is overall harmful for your health.[8] When nitrogen oxides interact with volatile organic compounds near the surface of the Earth, ground-level ozone is formed which can cause a variety of respiratory issues and exacerbate existing conditions like COPD and asthma which is common in vulnerable populations such as the elderly and children.[8] Lead is a soft metal that when introduced to the human body, can cause damage throughout many body systems including the nervous, cardiovascular, immune, and renal systems.[8] Carbon monoxide is introduced into the air through combustion reactions, and when inhaled, the carbon monoxide mimics oxygen molecules and binds to hemoglobin, not allowing for proper oxygenation, which can exacerbate pre-existing conditions such as asthma and COPD.[8] Sulfur dioxide and nitrogen dioxide both are synthesized through combustion reactions and have negative impacts on the respiratory systems.[8] All six of the criteria air pollutants are created either directly or indirectly from combustion reactions of fossil fuels.[8]