Why do all garbage trucks smell similar?

I've lived in five countries and visited over 40. Wherever I go, I find that garbage trucks smell similar despite differences in what people eat and discard. It turns out this is because the same biological processes play out everywhere.

Most of the smell comes from anaerobic decomposition. Oxygen is quickly used up inside dumpsters and trucks. Bacteria then break down organic waste without oxygen. This produces a small set of powerful odor compounds.

The most important are sulfur compounds and amines. Hydrogen sulfide smells like rotten eggs. Mercaptans smell skunky and sewer-like. Amines such as putrescine and cadaverine smell like decaying meat. These chemicals are detectable at extremely low concentrations.

Methane is produced during this process, but it has no smell. Its presence simply signals that anaerobic decay is underway. The odor comes from trace compounds released alongside it.

Garbage smells similar everywhere because human waste streams are similar. The same microbes dominate. The same molecules overwhelm our sense of smell.