Vog is a form of air pollution that results when sulfur dioxide and other gases and particles emitted by an erupting volcano react with oxygen and moisture in the presence of sunlight. The word "vog" is a portmanteau of the words "volcanic" and "smog". Vog is created when volcanic gases, primarily oxides of sulfur, react with sunlight, oxygen, and moisture, resulting in sulfuric acid and other sulfates. Vog is made up of a mixture of gases and aerosols, which makes it hard to study and potentially more dangerous than either on their own. Vog is different from smog, which is formed largely from the incomplete combustion of fuel, reacting with nitrogen oxides and ozone produced from carbon monoxide by reactions with sunlight. Vog contains mostly sulfur dioxide and acid particles, in contrast to urban, industrial, and other pollution sources, which also contain additional toxic contaminants, such as ozone and hydrocarbons. Vog can cause headaches, watery eyes, sore throat, breathing difficulties, flu-like symptoms, and general lethargy. Vog can negatively affect human health and agriculture, and acid rain can contaminate household water supplies by leaching metals from building and plumbing materials in rooftop rainwater-catchment systems. Vog can be harmful, especially for those who live nearest to the volcano, and it is particularly risky for those who have chronic breathing problems such as asthma or COPD. To protect oneself from vog, it is recommended to stay indoors and use an air-conditioner, if possible, and keep the air conditioner on recirculation setting.