Ionization air purifiers may be making our homes and offices more unhealthy places when cleaning products leave the air smelling lemon-fresh, according to research in the 1 April 2007 issue of Environmental Science & Technology. Scientists at the University of California, Irvine, report that ozone emitted by these purifiers reacts with certain volatile organic compounds such as limonene, producing potentially harmful levels of particulate matter (PM).

“In earlier work we showed ionization air purifiers, which are meant to remove particles from the air, to be producers of ozone, which itself causes a range of breathing problems and perhaps reduced resistance to infections,” says Sergey Nizkorodov, an assistant professor of chemistry at Irvine. “Now we show this ozone reacts with limonene entering the air from cleaning products to produce more PM than these machines can actually remove.” Limonene is used to scent cleaning products.

The researchers placed an ionization air purifier in an office equipped with a standard air exchange system. An ozone generator—a type of purifier that releases ozone to oxidize and theoretically neutralize volatile pollutants—was used as a comparison. d-limonene was injected into the room periodically to achieve concentrations similar to those encountered in offices after cleaning.

The ionization air purifier initially increased the ozone concentration by 5–15 ppb from a background level of 5 ppb, while the ozone generator raised it by some 250 ppb, according to Nizkorodov. With either machine operating, the limonene injections were accompanied by a spike of one to two orders of magnitude in the air’s PM2.5 and PM0.1 content—from 103 to 105 particles per cubic centimeter—that decayed over the next hour. “This means that in [limonene’s] presence, these air purifiers are actually air contaminators,” says Nizkorodov.

The researchers also produced a kinetic model to predict the net amount of PM produced by different machines under different indoor scenarios. “The results obtained with the model fitted the experimental data very well,” says Nizkorodov.

“The ultrafine particles produced by the reaction between volatile organic compounds and emitted ozone from ionization indoor air purifiers are of considerable interest since they are known to trigger oxidative stress and inflammation in the lungs,” says Claire Infante-Rivard, a professor of epidemiology at McGill University in Montréal. “The results of the present study should attract the attention of public health authorities regarding the safety of these devices.”

Peggy Jenkins, manager of the Indoor Exposure Assessment Branch at the California Air Resources Board, says that agency is currently developing a regulation to limit ozone emissions from indoor air-cleaning devices, especially ozone generators. “The Irvine results further support the need for this regulation,” she says. “In addition to reducing the directly emitted ozone and its associated impacts, the regulation will reduce the likelihood of particle and formaldehyde formation when limonene or other terpenes are present indoors.”

根據《環境科學技術》2007年4月1日一期上的研究，如果清潔產品在空氣中留下檸檬香味，離子空氣凈化器可能使我們的家庭和辦公室環境更加不利于健康。加利福尼亞大學歐文分校的科學家們說，這些凈化器釋放的臭氧同某些揮發性有機物——如檸檬烯（苯烯），發生反應，產生可能達到有害水平的顆粒物（PM）。

“早先的研究表明，用來清除空氣中顆粒物的離子空氣凈化器會放出臭氧，而臭氧本身就會導致各種呼吸問題，還可能降低人體對疾病的抵抗力，”歐文分校的化學系助教謝爾蓋·尼克羅多夫說。“現在我們的研究表明，這些臭氧同清潔產品釋放的檸檬烯發生反應，生成的顆粒物比凈化器實際能清除的還要多。”清潔產品中用檸檬烯來增加香味。

研究人員把一個離子空氣凈化器放在一間配備有標準空氣交換系統的辦公室里，再用一臺臭氧發生器作對照——臭氧發生器釋放臭氧進行氧化，理論上可以中和揮發性污染物。屋中定時噴射D-檸檬烯，產生類似辦公室清潔后的濃度。

尼克羅多夫說，離子空氣凈化器起初將臭氧的自然背景值從5ppb提高了5～15ppb，而臭氧發生器將臭氧濃度提高約250ppb。兩臺機器只要有一臺運行，隨著檸檬烯噴射，空氣中PM2.5和PM0.1含量會出現一兩次激增——每立方厘米103到105個顆粒——過一個小時才衰減。“這意味著如果有檸檬烯，這些空氣凈化器實際上是空氣污染器”，尼克羅多夫表示。

研究人員還設計了一個動力模型，來預測不同機器在不同室內環境下生成的顆粒總量。尼克羅多夫說，“模型得出的數據同試驗數據非常吻合”。

“由揮發性有機物和離子室內空氣凈化器釋放的臭氧反應生成超細顆粒，這特別值得注意，因為已知它們可以引發肺部的氧化應激和炎癥。蒙特利爾麥吉爾大學的流行病學教授克萊爾·殷凡蒂說，“目前研究的結果應該引起公共衛生當局注意，關注這些設備的安全問題。”

加州空氣資源委員會室內空氣污染暴露評定組干事佩吉·詹金斯說，該機構正在制定條例來限制室內空氣清潔設備，尤其是臭氧發生器的臭氧排放。“加州大學歐文分校的研究結果為這一條例提供了進一步的證據”，她說。“除了減少臭氧的直接排放和與之相關的影響外，這一條例還將減少室內存在檸檬烯和其它萜烯時顆粒物和甲醛形成的機會。“