Residents in one of China’s most polluted cities are breathing in smog particles that are harder than steel, according to Chinese researchers. But medical experts said the size of the particles – and not their hardness – was the main concern for human health. A team of scientists from Xian Jiaotong University’s school of material science and engineering collected samples of smog particles around Xian, in northwestern China’s Shaanxi province, in the winter of 2013, when the city was blanketed in pollution. Xian, home to 8.7 million people, has some of the worst air pollution in the country, last year ranking 374 among 387 cities monitored for air quality, official figures showed. “We wanted to figure out what formed smog particles,” Liu Boyu, a researcher from the university, said. “Since we are researchers on the strength of materials, we wanted to see how hard smog particles were.” They found the smog’s components varied across the city, and included chromium, iron, aluminium, and lead. Signs of smog rebound in China’s heavy-industry heartland as winter cuts expire They also found that the particles came in different shapes, with some like balls and others threads. But Liu said the biggest surprise was just how hard most of the particles were. About 70 per cent of the particles were hard enough to cause wear in most industrial machines made from alloys, he said. “They’re so hard they could even cause damage to precision machinery,” Liu said. Liu and his team presented the findings at the university on Friday as part of a summary of the school’s broader research over the last five years. Health experts said the hardness of the pollutants was not necessarily a factor when it came to respiratory health. Zhang Xin, from the respiratory department of Shanghai’s Zhongshan Hospital, said the biggest concerns for respiratory doctors were the size of the smog particles and their chemical make-up. “The seriousness of the damage caused by the materials we inhale depends on how far the particles can penetrate our body and what elements they comprise,” Zhang said. “The tinier the things we breathe, the farther they can go into our lungs, making it more difficult for us to expel, by spitting and other means.” China drawing up ‘fresh battle plan’ to tackle smog Xian’s poor air quality is exacerbated by geography and weather. The city, also known as the home of the terracotta warriors, sits in a basin between the Loess Plateau and the Qin Mountains, sheltered from the strong winds and heavy rains that help dissipate harmful particles in the air. Government data shows that over the past two years the quantity of PM2.5 in Xian has risen to 73 microgrammes per cubic metre, up from 61 recorded in 2014. World Health Organisation guidelines label any level over 25 microgrammes per cubic metre over a 24-hour period as unhealthy.