The build up of urine and feces where the hens live create ammonia that "irritate the birds' eyes and upper respiratory tract."(1)

After the hens no longer produce enough eggs, they are slaughtered. many hens are injured or killed during transportation to the slaughterhouse.

Footnotes
1. Dr. Ian Duncan, Professor of Poultry Ethology, University of Guelph, in The State of Poultry Welfare in Canada (Ottawa: Animal Action, 1996).
2. Canada Poultryman Magazine, January 1992.