The air in Tibet is much thinner, with oxygen partial pressure being 35% to 40% below that at sea level. Special passenger carriages are used, and several oxygen factories were built along the railway. Each seat in the train is equipped with an oxygen supply outlet for any possible emergency. The Chinese government claimed that no construction workers died during the construction due to altitude sickness related diseases. The railway passes the Kunlun Mountains, an earthquake zone. The 7.8 Kunlun earthquake struck in 2001 (but caused no fatalities). Dozens of earthquake monitors have been installed along the railway.