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.”

根据《环境科学技术》2007年4月1日一期上的研究，如果清洁产品在空气中留下柠檬香味，离子空气净化器可能使我们的家庭和办公室环境更加不利于健康。加利福尼亚大学欧文分校的科学家们说，这些净化器释放的臭氧同某些挥发性有机物——如柠檬烯（苯烯），发生反应，产生可能达到有害水平的颗粒物（PM）。

“早先的研究表明，用来清除空气中颗粒物的离子空气净化器会放出臭氧，而臭氧本身就会导致各种呼吸问题，还可能降低人体对疾病的抵抗力，”欧文分校的化学系助教谢尔盖·尼克罗多夫说。“现在我们的研究表明，这些臭氧同清洁产品释放的柠檬烯发生反应，生成的颗粒物比净化器实际能清除的还要多。”清洁产品中用柠檬烯来增加香味。

研究人员把一个离子空气净化器放在一间配备有标准空气交换系统的办公室里，再用一台臭氧发生器作对照——臭氧发生器释放臭氧进行氧化，理论上可以中和挥发性污染物。屋中定时喷射D-柠檬烯，产生类似办公室清洁后的浓度。

尼克罗多夫说，离子空气净化器起初将臭氧的自然背景值从5ppb提高了5～15ppb，而臭氧发生器将臭氧浓度提高约250ppb。两台机器只要有一台运行，随着柠檬烯喷射，空气中PM2.5和PM0.1含量会出现一两次激增——每立方厘米103到105个颗粒——过一个小时才衰减。“这意味着如果有柠檬烯，这些空气净化器实际上是空气污染器”，尼克罗多夫表示。

研究人员还设计了一个动力模型，来预测不同机器在不同室内环境下生成的颗粒总量。尼克罗多夫说，“模型得出的数据同试验数据非常吻合”。

“由挥发性有机物和离子室内空气净化器释放的臭氧反应生成超细颗粒，这特别值得注意，因为已知它们可以引发肺部的氧化应激和炎症。蒙特利尔麦吉尔大学的流行病学教授克莱尔·殷凡蒂说，“目前研究的结果应该引起公共卫生当局注意，关注这些设备的安全问题。”

加州空气资源委员会室内空气污染暴露评定组干事佩吉·詹金斯说，该机构正在制定条例来限制室内空气清洁设备，尤其是臭氧发生器的臭氧排放。“加州大学欧文分校的研究结果为这一条例提供了进一步的证据”，她说。“除了减少臭氧的直接排放和与之相关的影响外，这一条例还将减少室内存在柠檬烯和其它萜烯时颗粒物和甲醛形成的机会。“