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Your toilet knows what you did last summer.
Apparently, one way to estimate drug-use rates in a city is to tap into its sewers. The city of Oslo did this with what's called a Passive Sampler—filters that continuously collect samples of chemicals in the environment. After several weeks, old filters are replaced with new ones and the collection membranes are taken to a lab and analyzed with mass spectrometry. The same system has been used to look for pollution near oil derricks. But it can be configured to check for other substances, too.
For instance, in Oslo, levels of the drug ecstasy spiked in the city's sewer water, increasing 10 fold, during the two weeks when Norwegian teenagers celebrate high school graduation.