Chen Yuanyuan has a talent for identifying odors collected in special bags from landfills and industrial plants.

Sniffing garbage or pollutants often brings on an unpleasant reaction, yet it's an indispensable skill for her job as an environmental monitor.

Her nose is an important link in the chain of pollution control.

"It's easy to describe the job," she said. "The researcher smells the gas inside the bags, and tells you whether it's stinky or not."

Chen, 33, has been working at the Beijing Municipal Environmental Monitoring Center for around 10 years.

Her olfactory acumen provides important clues that help the watchdog and law enforcement go after polluters. Every step, from collecting to smelling, needs to be accurate, she said.

Researchers from the center collect gases discharged from polluting companies, garbage landfills, sewage treatment plants and other sources with special collecting containers every two hours from four monitoring sites.

Next, six researchers in a laboratory at the center inhale the gases at the same time, and smell three bags, one with the gas being tested and the other two with clean air. Chen designed the basic process.

"We dilute the gases at first to make them harmless, but they're always so stinky that they make us nauseous," she said.

Next to the smelling lab is a washroom in case anyone needs to vomit.

The gases can be kept for 24 hours at most, thus the sniffers need to do the tests as soon after samples are collected, no matter how late.

In 2006, the center received a complaint saying a pharmaceutical factory had illegally discharged malodorous gases. Chen and her colleagues waited until nighttime, finally collecting samples at around 1 am.

"We came to the lab and called up six researchers to conduct the smell tests at around 3 am," she said. They had given their results to the environmental watchdog.

The researchers avoid sniffing longer than three hours at a stretch to protect their sense of smell. It's part-time job for them, she said.

"Staff members in our monitoring center can do the smelling jobs after they pass a national test in this field," said Chen Wei, 33, another researcher with rich experience in smelling stinky gases.

People aged 18 to 45 years old can take the tests of such things as basic knowledge about odors and identifying a number of basic scents - for example, flower fragrances and sweat odors. If they pass, they might be chosen to join the sniff patrol, Chen Wei said.

It's different from identifying perfumes. The standards are higher and job is more pleasant, a researcher said: "What we do is important to environmental monitoring, and we're proud of that."