Unlike the protective ozone high in the atmosphere, ozone near Earth's surface is a killer. Exposure to the gas, which forms when sunlight cooks car exhaust and other pollutants, erodes the lungs and can trigger deadly asthma attacks. The new Ozone Web site from the European Environment Agency in Copenhagen, Denmark, displays hourly ozone measurements from more than 500 recording stations across Europe. Visitors can also compare current readings to daily, weekly, and monthly trends. The information, some of which is newly available in real time, allows the public to find out when concentrations are unsafe. The data can also help scientists who are trying to understand ozone's movement—within a day, it can wander 400 to 500 kilometers from its source.