The study found that “Among asthmatics, 38% report irritation from scented products, 37.2% report health problems from air fresheners, and 13.6% report their asthma was caused by toxic exposure.” Toxicology and Industrial Health 2009; 25: 71–78 (Asthma and Chemical Hypersensitivity).
Caress, SM, Steinemann, AC (2009) Asthma and chemical hypersensitivity: prevalence, etiology, and age of onset. Toxicology and Industrial Health 2009; 25;71. Sage Publications. PDF on University of Washington Libraries Website.