Plant pathology is the study of plant diseases. The most common causes of plant diseases are bacteria, fungi, algae, viruses, and roundworms. Plant diseases can drastically affect a country's economy. They may be responsible for the loss of up to ten percent of human crops each year. Plant pathology is also important in environmental studies. Many plants are especially sensitive to pollutants in the air and water. They can be used as "early warning systems" of pollution problems that may affect humans as well. For example, ozone at a concentration of only 0.005 parts per million produces noticeable changes in tobacco plants.