题目

 Exposure to environmental tobacco smoke (ETS)-also called “second-hand smoke” and “passive smoking” contains over 4 000 chemicals, including over 50 known carcinogens (致癌物) and many poisonous things. No safe level of ETS exposure has been proved. Nor is there any expectation that further research will identify such a level. Several recent reviews have confirmed the serious risks to health and life associated with passive smoking. Long time exposure to second-hand smoke has been proved as a cause of many of the same diseases caused by active smoking, including lung cancer, cardi用化学符号填写：2个氢分子 ；2个钙原子 ；氢氧化镁中镁显+2价 ；5个亚铁离子 ；所有银原子的总称 硫酸根离子 ； 铵根离子 ．