Suspended particulate matter found in urban areas where air pollution is severe may be a factor that leads to lung tumors, according to the results of a recent study announced Friday. The study, conducted by researchers at the Tokyo Metropolitan Research Laboratory of Public Health, used lab rats to test the effects of SPM on respiratory systems. Experts said the latest results will provide vital information for taking steps to reduce SPM in the air. The team, headed by senior researcher Kenichi Oyama, collected SPM along Tokyo's main roads using an air filter. He later mixed the particles into a saline solution. The solution was then introduced into the respiratory systems of some rats. The animals injected with SPM were kept for two years and the development of their lungs was compared to a control group of rats who were given pure saline solution during the same time span. Only two of 35 rats not exposed to SPM showed abnormalities in their lungs, such as benign tumors and other symptoms that can be early signs of cancer. However, 14 of the 34 rats injected with the saline solution mixed with SPM developed such symptoms, according to the study. Although there was no difference in the percentage of rats that eventually developed lung cancer, rats exposed to SPM were more prone to develop benign tumors, according to the study. The relationship between SPM, especially diesel emission particles, and human health has been the focus of several studies in recent years.