The mainland's notorious smog has spread from major city clusters around Beijing, Shanghai and Guangzhou to inland cities in central and western provinces and in the country's north. Several western cities including tourist hot spots Lhasa and Xian have, unusually, been shrouded in smog or dust for the last two days, while three cities in coastal Shandong province were also blanketed by smog yesterday as pollution readings went off the scale. "This is alarming. This shows China's unsustainable growth mode is taking its toll across the whole country," said Li Bo , a senior adviser at Friends of Nature, a mainland environmental organisation. Sandstorms hit Lhasa, 3,700 metres above sea level, on Thursday and yesterday, reducing visibility in some areas to less than five kilometres and disrupting flights at the city's Gonga airport. Video: A view of Beijing's smog from atop the Forbidden City Xinhua said weather forecasters expected the sandstorm would last till the weekend. It said a sharp drop in temperature, lower atmospheric pressure and accumulated airborne particles were mainly to blame. Air quality readings were off the scale on both days, with larger PM10 particles - those up to 10 microns in diameter - a major problem. As the Potala Palace and nearby snow-covered mountains disappeared into the dusty haze, some travellers were shocked that the city with the cleanest air on the mainland was affected. "Even Lhasa, the sacred city known for its crystal clear sky, succumbs to haze now. Are we losing the last piece of 'pure land'?" asked a weibo user. Some environmentalists worried that rapid expansion of mining activities near Gonga mountain was to blame for the sandstorms. Mine blasting could send particles into the air, according to Gabriel Lafitte, an Australian researcher. Xian and Baoji in Shaanxi, and Liaocheng, Dezhou and Jining in Shandong were also among the worst affected cities yesterday, with pollution readings off the charts. The mainland's persistent smog problem seems to have gone from bad to worse recently as the "airpocalypse" - a term coined earlier this year when Beijing suffered severe pollution - affects more cities. This month alone, pollution has been off the charts in at least 25 cities, sometimes for consecutive days. Only three of them were in the Beijing-Hebei-Tianjin cluster of cities known as the mainland's smog belt.