打扫后满室柑橘、草木清香，很多人下意识觉得房间一尘不染。但一项普渡大学的研究提醒我们：这种令人舒心的香味背后，正在发生看不见的化学反应，短短几分钟，就会生成大量纳米微粒，被我们吸入肺中。洁净的空气，本不该有浓郁香气。
A room that smells like citrus, pine, or flowers is often associated with cleanliness. Those familiar scents, however, can signal chemical reactions taking place in the air.
Research led by Brandon Boor at Purdue University found that fragrance compounds released by both conventional cleaners and botanical essential oil-based products can rapidly react indoors and generate nanoparticles. If inhaled, some of these extremely small particles can penetrate deep into the lungs.
The researchers say exposure can be reduced by choosing unscented products, improving ventilation with exhaust fans or open windows, and avoiding devices that generate ozone while scented cleaning products are being used.
The researchers presented their results at the fall meeting of the American Chemical Society (ACS) during the "Healthy Indoor Spaces: Bridging the Microbiome and Chemistry" symposium in McCormick Place.
"Importantly, cleaning removes viruses and bacteria from surfaces, but it can also generate invisible air pollution. There's no visible dust or smoke in the air, but these particles are forming." -- Brandon Boor
Cleaning Can Create Invisible Nanoparticles
"We showed that indoor ozone reactions with fragrances from cleaning produce nanoparticles that carry a respiratory dose comparable to, or greater than, what you would experience from standing outside along a busy road," says Boor, an Assistant Professor of Civil and Construction Engineering at Purdue University who studies indoor air quality. "The particles are different in terms of their composition, but the total dose can be higher. You're not seeing smoke, dust, or haze in the air. Instead, you think the air smells great so it must be clean."
Boor began investigating how cleaning agents and chemical disinfectants affect indoor environments during the COVID-19 pandemic with his colleague Nusrat Jung, a Purdue Assistant Professor of Civil and Construction Engineering. One feature quickly stood out: many of the products people use to clean and disinfect indoor spaces contain strong fragrances.
"That's often to create a pleasant smellscape in the indoor space," says Boor. "But clean air should not smell like highly concentrated citrus fruit. It should not really smell of anything."
Scientists who study the atmosphere have long known that compounds released by plants can react with ozone. Terpenes such as pinene from pine trees, for example, can participate in reactions that generate tiny airborne particles. Over time, those particles can combine and grow until they become large enough to help seed clouds.
That chemistry proceeds relatively slowly in forests because the amount of terpenes in outdoor air is generally low.
Cleaning Products Release High Levels of Terpenes
Indoors, the situation can be very different. Scented cleaners release terpenes when fragrance compounds evaporate from sprayed droplets or cleaned surfaces.
Cleaning liquids commonly contain compounds including pinene, limonene (lemon), thymol (thyme), and linalool (lavender). Their concentrations during cleaning can be far higher than levels typically measured outdoors. Boor says airborne terpene concentrations inside a room during cleaning can rise to tens or even hundreds of times those found in a forest.
To examine what happens under realistic conditions, the researchers tested scented conventional liquid products as well as botanical-containing disinfectant sprays and wipes inside a model home on Purdue's campus.
The small house includes a functional kitchen, wood flooring, and a bathroom, allowing the researchers to recreate ordinary household cleaning activities. Their experiments showed that the same basic chemistry responsible for nanoparticle formation outdoors can occur indoors much more rapidly and at much higher concentrations, with potentially important consequences for human exposure.
Billions or Trillions of Particles Can Form
Routine tasks such as mopping floors, spraying countertops, and wiping surfaces with scented products generate billions or trillions of particles, with the total depending on the product.
Most were nanoparticles or ultrafine particles measuring only 1-30 nanometers across. Because particles this small often fall outside the detection range of at-home air quality monitors, people may have no indication that particle concentrations have risen.
The researchers found that ordinary cleaning can temporarily push ultrafine particle levels above those measured outdoors.
Their tiny size is important from a health perspective. Ultrafine particles can settle throughout the respiratory tract and reach deep regions of the lungs. Once there, they can contribute to irritation and inflammation in the respiratory system. Some may also have the potential to enter the bloodstream.
One of the biggest surprises was the speed of the process. Particle formation and growth occurred within just minutes.
"By the time you finish cleaning up an indoor space, you've already formed a lot of nanoparticles and inhaled them," says Boor.
Ozone Can Intensify Indoor Particle Formation
More recently, Boor and Ernest Blatchley, a Professor at Purdue, studied what happens when scented surface cleaners are used at the same time as germicidal far-UV (UV-C) lamps designed to disinfect indoor air.
The combination created particularly favorable conditions for nanoparticle formation.
The lamps interact with oxygen in the air and produce ozone. During experiments in the tiny home, ozone concentrations increased to roughly 20 to 40 parts per billion. Those levels were comparable to, although somewhat below, the outdoor ozone concentrations measured when the experiments were performed.
With both elevated ozone and high concentrations of terpenes present, nanoparticle production became even more intense. That combination raised additional concerns about the amount of particulate matter occupants could inhale.
How to Reduce Exposure While Cleaning
Boor emphasizes that the goal is to help consumers make informed choices rather than discourage cleaning. Cleaning remains important for removing viruses and bacteria from surfaces, but several simple measures may reduce exposure to the secondary pollution produced during the process.
The researchers recommend:
Choose low-fragrance or fragrance-free products.
Avoid applying several scented products in the same cleaning session.
Run exhaust fans or open windows to ventilate the space.
Do not simultaneously clean surfaces with scented products while using ozone-generating devices, such as far UV-C lamps.
"Importantly, cleaning removes viruses and bacteria from surfaces, but it can also generate invisible air pollution," says Boor. "There's no visible dust or smoke in the air, but these particles are forming."
Boor thanks all the graduate students who have worked with him in the tiny house experiments along with the support of undergraduate students.
The research was funded by a National Science Foundation Faculty Early Career Development Program (CAREER) grant and the Alfred P. Sloan Foundation.
柑橘、松树或是花香萦绕的房间，往往会让人觉得洁净。然而这些熟悉的香气，其实意味着空气中正在发生化学反应。
普渡大学布兰登·布尔领衔的研究发现，普通清洁剂与含植物精油产品释放的芳香化合物，会在室内快速发生反应，生成纳米颗粒。这类极微小颗粒一旦被吸入，部分能够深入肺部。
研究人员表示，可以通过选用无香产品、开启排风扇或开窗通风、不要在使用芳香清洁剂的同时开启会产生臭氧的设备，来减少这类污染物的吸入。
该研究成果于美国化学学会（ACS）秋季会议，在麦考密克会展中心举办的“健康室内空间：打通微生物组与化学”专题研讨会上公布。
布尔说：“重点在于，清洁可以清除物体表面的病毒与细菌，但同时也会产生看不见的空气污染。空气中看不到粉尘、烟雾，可微粒正在悄然生成。”
清洁，会制造看不见的纳米微粒
“我们实验发现，室内臭氧和清洁剂中的芳香物质发生反应，生成的纳米颗粒，给呼吸道带来的吸入剂量，等同于甚至高于站在繁忙马路边所接触到的污染。”普渡大学土木与建筑工程助理教授布尔介绍，他长期研究室内空气质量，“颗粒成分虽不一样，但总吸入剂量可能更高。你看不见烟尘、雾霾，只会觉得气味清新，便认定空气干净。”
新冠疫情期间，布尔与同事努斯拉特·荣格（普渡大学土木与建筑工程助理教授）开始研究清洁剂、消毒剂对室内环境的影响。他们很快注意到一个现象：人们居家清洁消毒常用的很多产品，都添加了浓郁香精。
布尔解释：“添加香精，常常是为了营造宜人的室内气味。但洁净的空气本不该有浓烈柑橘味，理想状态下，空气几乎没有味道。”
大气科学领域的学者早已知晓，植物释放的化合物可与臭氧发生反应。例如松树中的蒎烯这类萜烯物质，参与反应后会生成空气中微小颗粒。久而久之，微粒不断聚合长大，足以成为云的凝结核。
在森林里，这类化学反应速度相对缓慢，因为室外空气中萜烯浓度通常很低。
清洁产品释放高浓度萜烯
室内环境则大不相同。芳香清洁剂喷洒后，香精成分会从雾滴或是擦拭过的表面挥发，释放萜烯。
清洁液中常见这类物质：蒎烯、柠檬烯（柑橘香气来源）、百里酚（百里香）、芳樟醇（薰衣草）。清洁时，室内这些物质的浓度，会远远高于室外自然环境。布尔提到，打扫房间时，空气中萜烯浓度可达森林中的几十甚至上百倍。
为模拟真实居家场景，研究团队在普渡大学搭建的实验样板小屋内，测试了含香精的传统清洁液、植物成分消毒喷雾与消毒湿巾。
这间微型小屋配有可用厨房、木地板与卫生间，可以复刻日常居家清洁行为。实验证实，户外能生成纳米颗粒的化学反应，在室内会以更高浓度、更快速度发生，给人体带来潜在健康风险。
可生成数以亿计甚至万亿级微粒
拖地、喷洒台面、用芳香湿巾擦拭等日常清洁行为，都会产生数十亿乃至上万亿微粒，数量取决于所用产品。
绝大多数是直径仅1–30纳米的纳米/超细颗粒物。家用空气检测仪大多无法捕捉如此微小的颗粒，人们很难察觉空气中微粒浓度已经升高。
实验发现，普通打扫，会短暂让超细颗粒物浓度超过室外马路边水平。
从健康角度，微小尺寸是关键特点。超细颗粒能够遍布整个呼吸道，直达肺部深处，引发呼吸系统刺激与炎症，部分颗粒还有可能进入血液。
最令人意外的一点是反应速度：短短几分钟之内，微粒就已经形成并持续增大。
布尔说：“等你打扫完房间，大量纳米颗粒已经生成，并且被你吸入体内。”
臭氧会加剧室内微粒生成
布尔与普渡大学教授欧内斯特·布拉奇利后续进一步研究：当使用芳香清洁剂，同时搭配用于空气消毒的远紫外UV‑C灯，会发生什么。
两者共存，会极大促进纳米颗粒生成。
紫外灯会和空气中的氧气作用，产生臭氧。在样板小屋实验中，臭氧浓度达到约20至40ppb，接近同期室外臭氧水平，略低一些。
高臭氧叠加高浓度萜烯，纳米颗粒的生成会变得更加剧烈。这种组合，让室内人员吸入颗粒物的风险进一步上升。
打扫时，如何减少污染物暴露
布尔强调：这项研究目的是帮助消费者理性选择，并非劝大家不要打扫。清洁对于清除物体表面病菌依旧十分重要，不过几个简单办法，可以减少清洁过程产生的二次污染。
研究团队给出建议：
1. 优先选淡香或者无香精清洁产品
2. 不要在同一次清洁中叠加多种芳香产品
3. 开启排风扇或者开窗通风
4. 不要一边使用芳香清洁剂擦拭物体，一边开启远紫外UV‑C等会产生臭氧的设备
“重点在于，清洁能够清除物体表面的病毒和细菌，但清洁过程本身，也会产生看不见的空气污染。没有粉尘烟雾，微粒却正在形成。”
布尔感谢所有参与样板小屋实验的研究生，以及本科生提供的协助。
本研究由美国国家科学基金会青年教师科研项目（CAREER）与阿尔弗雷德·P·斯隆基金会资助。
