These reactions got steadily more intense over a 20-year period. Friends' perfumes began to burn her eyes, bringing on searing headaches and dizzy spells. Vinyl ink and screen cleaners—products she used regularly for her art—started to clog her throat, leaving her wheezing and struggling to breathe.

Nicole Parisi had just moved to LA when she was first diagnosed with Multiple Chemical Sensitivity syndrome. As an artist, she had spent years coming into contact with harsh chemicals—regularly using paints, inks, and solvents for her screen print projects. Each time she handled them, though, she would notice a range of subtle bodily reactions.

One afternoon in 2003, Parisi passed out in her apartment building's laundry room. She'd inhaled a strong, flowery scent that overwhelmed her. The smell left her gasping, dizzy, and consumed by an instant and crushing headache.

But she hadn't smelled anything unusual or remarkable. The smell that overwhelmed her was simply the scent of standard laundry detergent. Life, she claims, hasn't been the same since.

"Since that day when I got knocked out from laundry fumes, every synthetic fragrance and most solvents [and] most paints, plastics, and exhaust makes me sick," she says. "I have difficulty breathing, seeing, speaking, thinking, walking, and I get terrible headaches lasting from two to 12 hours. Passing out and sleeping is the only remedy."

Parisi's story is no anomaly. Multiple chemical sensitivity—otherwise known as MCS or environmental illness (EI)—is one of the most mysterious health syndromes out there. Sufferers, up to 80 percent of whom are women, struggle with debilitating sensitivities to their surrounding environment, particularly when exposed to smoke, perfume, and cleaning products. (The exact chemicals that trigger these reactions vary, and depend on the person.) Essentially, sufferers of MCS become allergic to the modern world.

The syndrome appears to be prevalent in the US, where research suggests that between 12.6 percent and 15.9 percent of the population experiences some kind of "heightened sensitivity" to modern chemicals, with women being more susceptible. But information about its background and causes are rare. We know the common symptoms of MCS: confusion, cognitive dysfunction, asthma-type symptoms, rhinitis (inflammation of the nose), sleep disturbances, fatigue, anxiety, depression, according to one 2009 paper. But very little is known about what causes MCS, or why it's more common in women.

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Dr. Martin L. Pall, who has studied MCS extensively, thinks the answer lies in female hormones. "The best evidence on this gender ratio comes not from studies of MCS, but rather studies of the related disease, chronic fatigue syndrome (CFS)," he explains. "In CFS, sufferers who were diagnosed before puberty, the gender ratio is close to 1:1. In sufferers diagnosed after puberty, the ratio is close to four females: one male. That argues for a hormonal effect."