Guardian: Hu Li's heart sank when she realised that she could gauge how close she was to home by the colour of the air. Driving 140 kilometres from Tianjin City to Beijing last week, she held her breath as the chalky-white horizon became a charcoal grey haze. The 39-year-old businesswoman has lived in Beijing for a decade, and this past month, she said, brought the worst air pollution she has ever seen. It gave her husband a hacking cough and left her seven-year-old daughter housebound. "I'm working here and...