[chrissyb]

Not a doctor but.. You're falling for a bit of a naturalistic fallacy here. Just as Asbestos is a naturally occurring mineral however inhaling its dust can come with very serious health problems, so can hay contain mould which can also cause serious lung disease. http://www.ccohs.ca/oshanswers/diseases/farmers_lung.html

[IAMADR]

Chris is absolutely right, and in addition to mo(u)ld, there's also tons of antigens to provoke inflammatory responses in the lungs, which can lead to asthma and chronic bronchitis.