Carbon monoxide (CO) is a colourless, odourless gas that interferes with bodily oxygen delivery. It can cause headaches, dizziness, weakness, nausea, and even death. Potential sources include defective central heating furnaces, automobile exhaust, tobacco smoke, and space heaters using fossil fuels. The solution to CO contamination is source removal.

Humans and pets exhale carbon dioxide (CO2) as a natural part of the metabolic process. High levels of CO2 can cause drowsiness and headaches. ASHRAE recommends that indoor CO2 levels not exceed 700 ppm above outdoor ambient levels. The solution for elevated CO2 levels is ventilation.

Volatile organic compounds (VOCs) are chemicals found in home furnishings, carpets, building materials, paints and lacquers, cleaning supplies, waxes, pesticides, moth repellents, air fresheners, and dry-cleaned clothing. VOCs evaporate into the air when these products are used or sometimes even when they are stored. Volatile organic compounds irritate the eyes, nose, and throat and cause headaches, nausea, and damage to the liver, kidneys, and central nervous system. Some of them can cause cancer. Solutions for high VOC levels include ventilation and VOC destruction technologies.

Particulates can induce eye, nose, and throat irritation, respiratory infections, bronchitis, and lung cancer. Asthma triggers include mould, dust mites, secondhand smoke, pet dander, and other pollutants in the air. These triggers cause symptoms including coughing, chest tightness, wheezing, and breathing problems. Asthma attacks can be life-threatening. However, asthma can be controlled by reducing asthma triggers.

Moulds are living things that produce spores. These spores float in the air, land on damp surfaces, and grow. Inhaling or touching moulds can cause hay fever-type symptoms such as sneezing, runny nose, red eyes, and skin rashes. Molds can also trigger asthma attacks. Indoor humidity can increase the likelihood of mould. Indoor humidity should be kept between 30 and 50 percent.