Hu Li's heart sank when she realized that she could gauge how close she was to home by the color of the air.

Driving 140 km from Tianjin to Beijing last week, she held her breath as the chalky-white horizon became a charcoal-gray 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 7-year-old daughter housebound.

"I'm working here and my husband's working here, so we have no choice," she said. "But if we had a choice, we'd like to escape from Beijing."