But Zhang Liang, project manager at TUV Rheinland, an independent third-party inspection, testing and certification provider based in Germany, says there are still limitations to air purifiers.

"For example, most air purifiers on the market target formaldehyde, benzol and toluene in a house, but many other particles are not filtered by the machines, which can cause acute symptoms, such as coughing, sore throat and teary eyes, even chronic diseases."

However, engineers have not found any revolutionary way to solve this problem, other than installing a more powerful motor and adding more layers of activated-carbon filter inside the machine, experts say.

Zhang says more manufacturers are coming to TUV Rheinland to test their products, as people's awareness of quality grows.

TUV tests how effectively the machines absorb harmful pollutants that are prevalent in China, such as formaldehyde and coal dust.

"Three years ago, we received about three to four test requirements every year, but now we receive five requests every month," he says.

With China's pollution a long-term problem, Blueair is benefiting from the nation's demand for its purifiers. It now has 25 new products in the pipeline for China.

"The company is testing them and will introduce some to the Chinese market," Rittri says, adding that the Beijing and Shanghai offices are collecting feedbacks from users so they can understand Chinese consumers better.

"China may need some more years before the air becomes really clean."