NEW YORK — When a mysterious odor wafted through the city this week, Mayor Michael Bloomberg quickly appeared on television to reassure unnerved New Yorkers that the smell — whatever it was — was harmless.

The pronouncement was more than guesswork.

Over the past three years, the U.S. government has deployed hundreds of air-sniffing sensors in at least 30 metropolitan areas to create an early warning system for a chemical or biological attack. In some cities, the devices test the air 24 hours a day for traces of anthrax, smallpox and other deadly germs.

When the smell came through New York on Monday, it did not set off any of the system’s alarms. And that helped offer the public some reassurance.

Most of these urban monitoring networks are still in a fledgling stage, and authorities warn that they have their limitations. But the surveillance network has been steadily improving.

In some big cities, including New York, Boston and Washington, monitors have been installed in major train and subway stations to sample the air for poisonous chemicals or explosive gases. Also, environmental agencies have been given portable air sensors that can be driven around in vans or carried by hand.

Wireless technology may soon make it possible for the machines to do automatic testing and relay the results to a central monitor, eliminating the need to carry samples to a lab.

“Our objective is to make it an almost instantaneous result,” said Christopher Kelly, a spokesman for the Department of Homeland Security’s science and technology division.

The system has its limits. Among them, the devices are at the whim of wind patterns and can detect only substances that have already been released into the air — meaning that their primary usefulness is in getting victims treated quickly and preventing a contagion from spreading.

The sensors also cannot test for everything. The director of Homeland Security’s biological countermeasures program told Congress last May that the latest generation of the BioWatch system, the part of the sniffing network that monitors for deadly germs, will test for about 20 different microbes and toxins.

None of the many air-sampling systems available to investigators was able to actually identity the rotten-egg smell that wafted across parts of New York and New Jersey on Monday, officials said, and city investigators relied mostly on traditional methods of analyzing the odor, including handheld meters long used by utility crews to check for gas leaks.