Two vans loaded with precision instruments trundled along the streets of New York and New Jersey in the heat earlier this week, sniffing for toxic chemicals in the air. They detected spikes in methane, a potent greenhouse gas, most likely from leaks, or from natural-gas-burning buses. And all along the ride, they logged elevated levels of ozone, the main ingredient of smog, as well as cancer-causing formaldehyde — both of which form readily in hot weather. The bottom line: The streets are dotted with pollution hot spots. And the heat makes pollution worse.

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Persons: ”, Peter DeCarlo, Johns Hopkins University who’s Organizations: Johns Hopkins University Locations: New York, New Jersey