Potential risk factors were evaluated by multiple logistic regression analysis. For the independent variables, age and working years were included as continuous variables, residential areas and working areas as ordinal variables and those included as categorical variables were education (college graduate=3, high school=2, secondary school=1), smoking habit (ever=1, never=0), spouses’ smoking (ever=1, never=0), having the previous history of dust exposure (yes=1, no=0), having a cat, dog or bird in house (yes=1, no=0).