Multiple chemical sensitivity (MCS) is a chronic medical condition characterized by symptoms the affected person attributes to exposure to low levels of chemicals. Commonly suspected substances include smoke, pesticides, plastics, synthetic fabrics, scented products, petroleum products and paints. Symptoms may be vague and non-specific, such as nausea, fatigue, and headaches.

MCS is a controversial diagnosis, and is not recognised as an organic, chemical-caused illness by the American Medical Association. Blinded clinical trials have shown MCS patients react as often and as strongly to placebos, including clean air, as they do to the chemicals they say harm them. This has led some to believe MCS symptoms are due to odor hypersensitivity or are mainly psychological. Regardless of the etiology, some people with severe symptoms are disabled as a result.