The term "multiple chemical sensitivity (MCS)" refers to a medical condition in which an individual experiences adverse health effects from exposure to low levels of certain chemicals. People with MCS report a range of symptoms, including headaches, fatigue, and respiratory problems, when exposed to everyday substances such as fragrances, cleaning products, or even freshly cut grass. The exact cause of MCS is not well understood, but it is believed to involve an overactive immune system and a heightened sensitivity to chemicals in the environment.