Is spitting common?

Is it common for Chinese people to spit? Even indoors? I read a book by humorist David Sedaris, and one of his sketches was about a visit to some Chinese cities and how disgusted he was by all the spitting he was seeing. Is this a real thing? Or was it possible that he and his travel companions were evoking this behavior, possibly because they were violating social norms without knowing it? Fill me in.