Hong Kong's pollution levels hit nearly decade-level highs this week, sending locals scurrying inside and obscuring the city's skyline behind a blanket of white.

In the city's central business district, roadside monitoring devices clocked levels of pollution as high as 210 on the city's air pollution index yesterday, with high levels persisting through Tuesday afternoon. That figure is nearly the highest the city has experienced since 1999, with the exception of levels reached when a dust storm blew in from mainland China...