Formaldehyde is widely present, from the universe to a single cell. It comes from our daily life, including air, food, water, clothing, and building materials. As the main material or a byproduct of industrial and commercial production, formaldehyde is the most common aldehyde in our environment. Contaminated air is a usual source of formaldehyde exposure. In general, humans spend approximately 90% of their time indoors, where there are complex mixtures of pollutants, including formaldehyde. Indoor formaldehyde exposure can be extremely dangerous, such as residential exposure to renovated homes, offices, and public places such as shopping centers and barbershops. It is worth mentioning occupational formaldehyde exposure in some factories, hospitals, and laboratories, where formaldehyde concentrations are higher and the people working are vulnerable. At the same time, outdoor formaldehyde sources should not be neglected for air exchange and transfer to indoor formaldehyde sources. Although the toxicity of formaldehyde has been extensively studied and its detection methods have been well developed, exposure to formaldehyde is still serious all over the world, especially the countries that are producing and using huge amounts of construction materials that contain and emit aldehydes. People are exposed to formaldehyde through breathing, gastric intestinal digestion, or skin contact. Exposure to formaldehyde can cause respiratory irritation, asthma, tumors, and multiple neuropsychological abnormalities or even cause death. Therefore, it is important to control any possible sources of formaldehyde and set up international standards to avoid formaldehyde insult.