To the central government, the smog that blankets the country is not just a health hazard, it's a threat to national security. Last month visibility in Harbin dropped to below three metres because of heavy smog. On days like these, no surveillance camera can see through the thick layers of particles, say scientists and engineers. To the authorities, this is a serious national security concern. Beijing has invested heavily to build up a nationwide surveillance network that lets police watch every major street and corner in main cities. But with smoggy days becoming more frequent, the effectiveness of the system has been greatly compromised. Some fear terrorists may choose a smoggy day to launch attacks. Kong Zilong, a senior project engineer with Shenzhen Yichengan Technology and an expert in video surveillance technology, said the security devices that could function in heavy smogs had yet to be invented. Existing technology, such as infrared imaging, can help cameras see through fog or smoke at a certain level, but the smog on the mainland these days is a different story. The particles are so many and so solid, they block light almost as effectively as a brick wall. "According to our experience, as the visibility drops below three metres, even the best camera cannot see beyond a dozen metres," he said. His company sells products from some of the world's leading security camera makers, such as Raymax from Japan, Bewator from Britain, FLIR from the United States and VisSim from Norway. The government has come to realise the seriousness of the issue and commissioned scientists to come up with a solution. The National Natural Science Foundation of China funded two teams, one civilian and one military, to study the issue and has told the scientists involved to find solutions within four years. Professor Yang Aiping, an expert in digital imaging with the School of Electronic Information Engineering of Tianjin University and leader of the civilian team, said she was facing tremendous pressure because of the enormous technological challenges. "Most studies in other countries are to do with fog. In China, most people think that fog and smog can be dealt by the same method. Our preliminary research shows that the smog particles are quite different from the small water droplets of fog in terms of optical properties," she said. "We need to heavily revise, if not completely rewrite, algorithms in some mathematical models. We also need to do lots of computer simulation and extensive field tests." The military team is led by professor Bi Duyan of the Air Force Engineering University of the People's Liberation Army in Xian , Shaanxi province. Bi could not be reached for comment on the research. Professor Zhang Li, an image processing expert with the department of electronic engineering of Tsinghua University, said the researchers might have to think out of the box. "On the smoggiest days, we may need to use radar to ensure security in some sensitive areas," he said. Microwaves or electromagnetic waves could travel through smog easily and bounce back if they hit an object. With the help of good software, sharp and clear images could be produced. But a radar camera would also generate radiation that harms people's health. "It has to be a contingency device," Zhang said.