SINCE the 1940s, southern California has had a reputation for smog. Things are not as bad as once they were but, according to the American Lung Association, a health group, Los Angeles is still the worst city in the United States for levels of ozone, nitrogen oxides, hydrocarbons and carbon monoxide. Gazing down on the city from the Getty Centre, an art museum in the Santa Monica Mountains, haze can blot out the view of the Pacific Ocean. Nor is the state’s bad air restricted to its south. Fresno, in the central valley, comes top of the list in America for year-round particulate pollution. Residents’ hearts and lungs are strained as a consequence.

All of which, combined with California’s reputation as the home of technological innovation, makes the place ideal for developing and testing systems designed to monitor pollution in detail. And that is just what Aclima, a fledgling firm in San Francisco, has been doing over the past few months. It has been trying out arrays of monitoring stations, some of them mobile, that are intended to yield minute-to-minute maps of outdoor air pollution. Such stations will also be able to keep an eye on what is going on inside buildings, including offices.

The stations in Aclima’s arrays are triangular and measure 20cm along a side. Each contains 12 off-the-shelf sensors that detect pollutants such as ozone, carbon monoxide and methane, as well as the small particles of which smog is composed. They also monitor basic data such as temperature and humidity, which affect how pollution forms and lingers. The elements of an array may be fixed and spaced according to what exactly they are trying to measure. Or they can be attached to vehicles, so that entire roads can be sampled routinely.

To this end, Aclima has been collaborating with Google’s Street View system, resulting in maps such as the one below. Davida Herzl, Aclima’s boss, says they have, as expected, revealed pollution highs on days when San Francisco’s transit workers went on strike and the city’s inhabitants were forced to take to their cars. Conversely, “cycle to work” days have done their job by creating pollution lows.

Aclima has already, at Google’s expense, mapped 20,000km (12,000 miles) of roads in the Bay Area this way. It plans to produce a detailed map of the air quality of California’s most populous regions—those around Los Angeles, San Francisco and the Central Valley—before the year is out. It will then move on to the rest of America and (Ms Herzl hopes) beyond. For such outdoor work, most of its customers are likely to be charitable foundations or governments. But Aclima hopes to make money from the private sector as well, by measuring pollution levels indoors.