Cleanliness is generally perceived as being associated with health, and maintaining a clean home by reducing asthma triggers, such as dust mites and cockroaches, has been recommended as a "critical" component of asthma management (1). However, the article by Zock and colleagues in this issue of the Journal (pp. 735-741) presents results showing that how you clean your house may have adverse respiratory health effects (2). Analyzing data from 22 centers from 10 European countries that have participated in the clinical part II of the European Community Respiratory Health Survey, they found a statistically significant association between new onset of wheeze, nocturnal attacks of shortness of breath, use of asthma medication, and/or physician-diagnosed asthma in adults and the use of cleaning sprays in the home. Their findings are biologically plausible with the highest risk being among individuals with the greatest potential for inhalation exposure, namely, those using sprays in their homes at least four days a week and/or using more than three different types of sprays at least one day per week (2). Although this is the first study to demonstrate an association between the use of cleaning products in the home and the onset of asthma among adults, we should not be surprised by these results, which are consistent with previous knowledge about the adverse respiratory effects of cleaning products. First, there have been at least six previous epidemiologic studies as well as multiple case reports and case series describing increased asthma and respiratory symptoms among workers who clean homes or commercial and industrial facilities (3, 4). Second, each year cleaning agents are either the second or third most common exposure among adults who are reported to the American Association of Poison Control Centers Toxic Exposure Surveillance System (70,000-80,000 reports/year, 9-10% of all reports/year) (5). Third, cleaning agents contain many different types of chemicals, including known irritants and allergens. Studies have shown these chemicals in the indoor air of homes (6). Fourth, a recent study found a reduction in FEV1 and maximum mid-expiratory flow rate that was statistically associated with blood levels of 1,4-dichlorobenzene, a chemical found in air fresheners, toilet bowl deodorants, and mothballs (7). Finally, there is a previous report of increased risk of persistent wheezing in young children up to the age of 3.5 years in association with increased domestic household chemical use, and predominantly cleaning agents that were used during pregnancy (8). The methodology used by Zock and coworkers is strong; longitudinal design, large sample size, and standardized data acquisition. However, the study design does not allow the authors to assess the biological mechanism for the association. In addition, their results are equivocal on whether the adverse respiratory outcome they found was the new onset of asthma or respiratory symptoms without asthma. They reported associations between the use of cleaning products and wheezing or whistling in the chest in the previous 12 months with the use of cleaning products and physician-diagnosed asthma. They found no increased risk with the use of cleaning products for the development of wheeze in people with a positive methacholine challenge test, and only a nonstatistically significant association in people with a physician diagnosis of asthma with a positive methacholine challenge test (2). Four possible biological mechanisms, none of which are mutually exclusive, may explain the results of this study. First, high exposure to an irritant may cause reactive airways dysfunction syndrome (RADS) (9). Inappropriate mixture of a cleaning product containing bleach and a cleaning product containing an acid will release chlorine gas, and inappropriate mixture of bleach and ammonia will release chloramines. Second, chronic low-level exposure to the many irritants in cleaning compounds may cause inflammatory changes (10). Third, cleaning agents are known to contain sensitizers, such as amine compounds, benzylakonium chlorine, and disinfectants, such as chlorhexidine (3). There are no regulatory requirements for companies to test their products or ingredients for the ability to cause type 1 immunologically mediated reactions, such as asthma. Routine toxicological testing performed by companies that produce cleaning products includes assessing the irritant potential of the compound. Additional testing may sometimes include testing for contact dermatitis. Accordingly, the number of potential sensitizers that are present in cleaning products is unknown. Finally, although Zock and colleagues found no effect of atopy on their results, individuals who clean would be expected to have increased exposure to dust and dust mites. The ability of a chemical in a cleaning product to increase reactivity to an allergen such as dust mites has been reported in both animals and human studies (11). All four mechanisms could contribute to the epidemiologic findings of Zock and colleagues. Despite the uncertainty of the biological mechanism of the findings reported by Zock and colleagues, clinicians should be aware of the potential for cleaning products used in the home to cause respiratory symptoms and possibly asthma. The lack of pre-testing of cleaning products by industry has led to at least one recall of a miticide carpet powder and spray, marketed for individuals with asthma, after consumers reported asthma attacks and respiratory symptoms 15 to 30 minutes after use (12). As part of the settlement, the company provided funding to a nonprofit organization to purchase and staff a mobile asthma clinic. Obtaining information on the timing of symptoms and asthma exacerbations should be considered an important part of clinical management. Clinicians practicing outside of the United States, where access to specific antigen bronchial challenge testing is more readily available, will probably be able to identify new sensitizers present in cleaning products. Repeated peak flow testing by patients, as has been reported in one study of professional cleaners, may be useful in better defining both the cause and severity of symptoms (13). The ubiquitous use and exposure to cleaning products emphasize the importance of clinicians being aware of the potential for respiratory toxicity. In addition, there is the need for researchers to conduct further studies to elucidate both the extent and mechanism of the respiratory toxicity associated with such products. Finally, the industrial producers and governmental regulators must improve the toxicological testing of these products.