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Due to animal welfare considerations, there is an increasing interest in development of floor housing systems for laying hens as an alternative to cage systems. However, the hygienic threshold limit value of 25 ppm ammonia established for animal welfare (Swedish Board of Agriculture, 2003) and for occupational safety and health (Swedish National Board of Occupational Safety and Health, 2000) is often exceeded in floor housing systems for laying hens with long-time storage of manure in bins below draining floors (Mannebeck & Oldenburg, 1990; Gustafsson & Mrtensson, 1990; Hillig, 1992; von Wachenfelt, 1993; Hauser & Flsch, 1993; Koerkamp et al., 1995; Koerkamp, 1996). In some cases, concentrations as high as 80 ppm have been measured (von Wachenfelt et al., 2002).

High ammonia concentrations can cause health problems both among humans as well as among animals. Measures to reduce the ammonia concentration in the air in floor housing systems are therefore urgent. However, knowledge is still limited about how different factors influence the concentration levels and the release of ammonia in this type of housing system.