Parents who smoke often open a window or turn on a fan to clear the air of second-hand smoke, but experts now have identified another smoking-related threat to children’s health that isn’t as easy to get rid of: third-hand smoke.

That’s the term being used to describe the invisible yet toxic brew of gases and particles clinging to smokers’ hair and clothing, not to mention cushions and carpeting, that lingers long after smoke has cleared from a room. The residue includes heavy metals, carcinogens and even radioactive materials that young children can get on their hands and ingest, especially if they’re crawling or playing on the floor.

Doctors from MassGeneral Hospital for Children in Boston coined the term “third-hand smoke” to describe these chemicals in a new study that focused on the risks they pose to infants and children. The study was published in this month’s issue of the journal Pediatrics.

“Everyone knows that second-hand smoke is bad, but they don’t know about this,” said Dr. Jonathan P. Winickoff, the lead author of the study and an assistant professor of pediatrics at Harvard Medical School.

“When their kids are out of the house, they might smoke. Or they smoke in the car. Or they strap the kid in the car seat in the back and crack the window and smoke, and they think it’s okay because the second-hand smoke isn’t getting to their kids,” Dr. Winickoff continued. “We needed a term to describe these tobacco toxins that aren’t visible.”

Third-hand smoke is what one smells when a smoker gets in an elevator after going outside for a cigarette, he said, or in a hotel room where people were smoking. “Your nose isn’t lying,” he said. “The stuff is so toxic that your brain is telling you: ’Get away.’”

The study reported on attitudes toward smoking in 1,500 households across the United States. It found that the vast majority of both smokers and nonsmokers were aware that second-hand smoke is harmful to children. Some 95 percent of nonsmokers and 84 percent of smokers agreed with the statement that “inhaling smoke from a parent’s cigarette can harm the health of infants and children.”

But far fewer of those surveyed were aware of the risks of third-hand smoke. Since the term is so new, the researchers asked people if they agreed with the statement that “breathing air in a room today where people smoked yesterday can harm the health of infants and children.” Only 65 percent of nonsmokers and 43 percent of smokers agreed with that statement, which researchers interpreted as acknowledgement of the risks of third-hand smoke.

The belief that second-hand smoke harms children’s health was not independently associated with strict smoking bans in homes and cars, the researchers found. On the other hand, the belief that third-hand smoke was harmful greatly increased the likelihood the respondent also would enforce a strict smoking ban at home, Dr. Winickoff said.

“That tells us we’re onto an important new health message here,” he said. “What we heard in focus group after focus group was, ‘I turn on the fan and the smoke disappears.’ It made us realize how many people think about second-hand smoke — they’re telling us they know it’s bad but they’ve figured out a way to do it.”

The data was collected in a national random-digit-dial telephone survey done between September and November 2005. The sample was weighted by race and gender, based on census information.

Dr. Philip Landrigan, a pediatrician who heads the Children’s Environmental Health Center at Mount Sinai School of Medicine in New York, said the phrase third-hand smoke is a brand-new term that has implications for behavior.

“The central message here is that simply closing the kitchen door to take a smoke is not protecting the kids from the effects of that smoke,” he said. “There are carcinogens in this third-hand smoke, and they are a cancer risk for anybody of any age who comes into contact with them.”

Among the substances in third-hand smoke are hydrogen cyanide, used in chemical weapons; butane, which is used in lighter fluid; toluene, found in paint thinners; arsenic; lead; carbon monoxide; and even polonium-210, the highly radioactive carcinogen that was used to murder former Russian spy Alexander V. Litvinenko in 2006. Eleven of the compounds are highly carcinogenic.

吸烟的父母常常开窗或者开电扇，使室内通风，从而清除空气中的“二手烟”，但现在专家发现了另一种由香烟导致的难以治愈的儿童健康疾病：三手烟危害。

这个新词汇所指的是，房间里烟味消逝很久之后，吸烟者头发和衣物上仍携带着的不可见但有毒的气体和颗粒，不限于椅垫和毛毯上的气体和颗粒。这些包括重金属，致癌物资甚至放射性物资在内的残余物，沾满了婴幼儿的双手甚至被他们的呼吸道吸收。当婴幼儿在地板上爬行或者玩耍时，所受的污染尤甚。

在研究这些化学物资对婴幼儿的危害的过程中，波士顿婴幼儿大众医院的医生们量身度作了“三手烟”的这个词来描述这些化学物资。研究结果发表在这个月的小儿科杂志上。

“每个人都知道二手烟有害，但是他们并不知道‘三手烟’”，研究报告的主要作者和哈佛医学院的助理教授，小儿科乔娜珊-维尼沃夫说。

“当孩子们在屋外，父母们在屋外吸烟。或者父母们把孩子放在以窗子隔离的后座上，自己在车里吸烟。这些父母认为自己做的做法很好，因为孩子们无法吸到二手烟。”维尼沃夫继续说道，“我们需要一个词来表述这些看不见的香烟毒素。”

当吸烟者在外吸完一支烟走进电梯或时，旁人闻到的即是三手烟。走进有人正在吞云吐雾的房间，闻到的也是三手烟。维尼沃夫介绍说。“鼻子并不会撒谎，”他说，“这些气味非常有害，我们的大脑都在叫嚣‘出去’。”

这个研究报告的调查对象涵盖了全美1500个吸烟家庭，发现大多数吸烟者和不吸烟者都清楚知道二手烟有害。大约95%的不吸烟者和84%的吸烟者也同意“婴幼儿因吸入父母烟灰缸烟味而致病”的论断。

但是这些被调查者中，非常少的人知道三手烟的危害。这个词语新颖，所以调查者换了种说法0问人们是否同意“待在昨天有人抽过烟的屋子里对婴幼儿身体不好”的论断。只有65%的不吸烟者和43%的吸烟同意此说法。调查者即以这一比例作为公众对三手烟的认知度。

维尼沃夫医生说，调查者发现，屋内和车内严格的禁烟令与二手烟影响儿童健康这一认知密不可分。另一方面，三手烟有害这一认知，使得被调查者更有可能强化家庭内的禁烟行为。

“这告诉我们需要宣传一个重要的新健康信息。”他说，“我们不停的在受访者间听到‘我打开了电扇，烟味都被吹走了’，从这点可知，许多人都知道二手烟有害，他们告诉我们他们知道室内抽烟不好，并且找出了一个解决的办法。”

研究数据是在2005年9月至11月的全国随机电话调查中收集的。样本家庭在人口信息的基础上，遍布了不同种族和性别。

领导纽约塞纳毛特医学院的儿童环境卫生中心的儿科医生，菲利普-兰德里甘医生说，三手烟是一个崭新的行为学词汇。

“这词所表达的重点意义是，简单的关上厨房门吸烟并不是避免儿童远离香烟危害的办法，”他说，“三手烟里的致癌物资，对与吸烟者有联系的任何人都有潜在的危险性。”

三手烟中的物资包括用于制造化学武器的氢氰化物，轻质石油的丁烷，涂料稀释剂中的甲苯，砷，铅，一氧化碳，甚至还有谋杀了前苏联间谍亚历山大利维尼柯的极具放射性致癌物资钋210，三手烟中11种组成物质都是高致癌物资。