This equipment produces more carbon monoxide when engines don't burn cleanly. At high concentrations, carbon dioxide and carbon monoxide can cause respiratory distress and headaches. At prolonged high exposures, these gases can be fatal. These gases usually aren't a problem under normal conditions with proper ventilation. However, it's important to maintain some ventilation at all times, even when animals are not present.
Ammonia is a troublesome by-product of manure decomposition.
It is an eye irritant at levels above 20 to 25 ppm. When exposed to levels around 1,500 ppm, people begin to cough and froth at the mouth. Ammonia is deadly at 5,000 ppm. Ammonia has a distinct odor, which humans can detect in concentrations as small as 5 ppm. It's common during winter months for ammonia levels to exceed 25 ppm, even under normal winter ventilation rates.