The World Health Organization includes the International Agency for Research on Cancer (IARC). Its primary purpose is to discover the causes of cancer. The International Agency for Research on Cancer defines diesel engine exhaust as "carcinogenic to humans" based on adequate evidence that it is associated to an increased risk of lung cancer. However there is no evidence that indicates that other types of cancer are caused by vehicle emissions. Emissions from cars and trucks account for a large portion of air pollution in large cities around the world. Evidence shows that car emissions contain harmful substances that can lead to cancer. Diesel engines emit nitrogen oxides (NOx) into the atmosphere, some of which combine with moisture to form fine particles that can be breathed in. These particles can enter the lungs and blood stream and over time may develop into cancers such as bronchitis or lung cancer. Emissions from gasoline engines are also toxic but not as much as those from diesel engines. A study conducted by the University of California, Berkeley found that drivers in Silicon Valley exposed themselves to high levels of benzene, a known carcinogen, for several hours every day when their vehicles were parked at work or home. The researchers concluded that "the chronic exposure to low levels of benzene appears to be enough to increase one's risk of developing leukemia." They added that "more research is needed to determine how often this phenomenon occurs in different parts of the country and whether it applies to other types of cancer.""