On days when Beijing’s heavy air pollution is especially pungent, you can smell and taste the acridity—whether you’re outside on the street or inside most buildings. Air pollution doesn’t stay outdoors but seeps inside through open doors and window sealings. On most days, levels of dangerous pollutants, such as PM 2.5, are somewhat lower outside than inside, but not much lower.
This unhappy fact has fueled a growing market for pricey indoor air filters in China, made by such companies as Chicago’s BlueAir and Switzerland’s IQAir. A basic model will set you back at least $800. And ideally, you should have one for each room in your home, school, restaurant, or office. In other words, these filters don’t come cheap.