Ammonia (NH3) is an invisible compound gas. Cat litter is highly absorbent and can, therefore, contain ammonia even from small amounts of urine & bacteria. The biological activity of the urine and bacteria in the litter box continually generates ammonia, and regenerates ammonia, so it continually builds up in the litter box over time. Larger quantities of urine and bacteria (for example from multiple cats) can accumulate even higher over many days/weeks can pose a health risk, not only to cats but also humans; especially to the elderly, young children, pregnant women and persons with compromised immune systems or already developed respiratory ailments, like asthma.
A cat may live upwards of 20 years, thus exposing it to years of using a litter box. Respiratory exposure is the most common route of ammonia exposure. If inhaled, ammonia can cause acute symptoms such as headaches, coughing, sore throat, dizziness, runny or burning nose, and a burning respiratory tract. Higher concentrations of ammonia exposure can cause bronchial conditions (such as shortness of breath, pneumonia, and asthma), pulmonary edema, and in severe cases, death in cats and humans.