Smoke irritates the eyes, nose, and throat, and the odor can be unpleasant. Inhaling small particles can have a number of negative health impacts, including respiratory discomfort and shortness of breath, and can aggravate medical disorders including asthma and heart disease. Smoke contains carbon monoxide, which can bind to hemoglobin in blood cells, preventing it from carrying oxygen to tissues cells. This can lead to dizziness, headache, loss of consciousness, and possibly death if left untreated. Fires can emit toxic chemicals in addition to smoke. These include carbon monoxide, nitrogen oxides, and fine particulate matter. All fire emissions are harmful to some degree, but with the exception of water vapor and sulfur dioxide, most are also human-made substances that could be avoided by using alternative methods to control buildings or vehicle engines for heat or power. The main way people become sick from fire smoke is by breathing it in. Airborne particles smaller than 2.5 microns (millionths of a meter) can get deep into the lungs where they can cause problems for people with asthma or other lung diseases. Long-term exposure to high levels of smoke particles may also lead to heart disease, cancer, nerve damage, reproductive issues, and eye problems. People who live near commercial fires or industrial sites that use fossil fuels as their source of energy may be exposed to higher levels of smoke pollutants than others.