Welcome to Beijing. Don’t worry, the air isn’t always this bad! AQI is the US’ “Air Quality Index,” while API is China’s “Air Pollution Index.” Both are 0-500 numbers that indicate the air quality at a certain time, although the definitions are slightly different. When it comes to particulate pollution (Beijing’s dominant air quality problem), though, the scales are similar, so it is somewhat reasonable to compare them directly. The biggest difference is that the US’ AQI measures only the smallest, most damaging particles to human health, while China’s API includes larger particles. I wrote some about it in this post: http://www.livefrombeijing.com/2008/12/stay-inside-today-beijing-api-at-246/.

Some other resources are as follows:
US’ AQI basics: http://www.airnow.gov/index.cfm?action=aqibasics.aqi
US’ AQI for particulates: http://www.airnow.gov/index.cfm?action=pubs.aqguidepart
Explanation of China’s API: http://www.livefrombeijing.com/2008/08/what-is-the-api-and-how-is-it-calculated/

Best,

Vance