​San Francisco is known for its progressive environmental policies. So, it comes as no surprise that the city has invested in a sensor-based solution to monitor the quality of its air. The real-time measurement system tracks pollutants carbon dioxide (CO₂), nitrogen dioxide (NO₂), ozone (O₃), and fine particles, plus gas-leak indicator methane (CH₄). The data collected by the sensors is used to generate pollution maps. The system is powered by a technology developed by CEA-Leti, a CEA Tech institute, and commercialized by startup eLichens.

The low-cost, low-power, miniaturized optical CO₂ and CH₄ sensors were developed by Leti. A silicon MEMS made up of a suspended structure that emits infrared (IR) radiation when it is heated to 650 °C is packaged inside a vacuum cap, also fabricated on the silicon substrate. The gases are held inside a chamber, where the IR rays emitted by the source pass through them. The light transmitted is filtered to select the wavelength that corresponds to the gas to detect and measure. The energy loss (absorbed by the gas) is proportional to the concentration of CO₂ and CH₄ in the chamber.  

The technology used to fabricate the IR source was transferred to a foundry; but Leti is still working with eLichens via a joint R&D lab to develop new sensors.