热度 26
小剂量放射线对人体有利还是有害？
大剂量辐射会造成人体健康危害是非常确定的，但是人们受到大剂量辐射的可能性比较小，而受到小剂量电离辐射影响的机会非常多，人们在进行医疗检查、飞行和工业生产过程中，经常会接受小剂量辐射的照射。不过，小剂量电离辐射对人体健康到底是否有不利影响，目前学术上并没有形成共识。最近美国科学家敦促美国能源部关注这一问题。1月7日，美国众议院通过一项法案，试图改变和推动现有能源部低剂量辐射研究项目。
关于小剂量辐射损伤的问题，早在半个世纪前就存在争议。一个非常有名的关于衰老的理论就是基于放射生物学的研究，因为辐射对机体损伤的基础是能促进自由基的产生，所以衰老的自由基学说就是Denham Harman在1956年根据辐射导致自由基增加的研究提出的。他开始认为，人所以会衰老，是因为环境中的小剂量辐射导致人体自由基产生增加，自由基产生的危害并逐渐积累导致衰老。后来发现，自由基产生并不是辐射这一种原因，各种生物过程都会产生自由基，才逐渐修改了这种理论。无论如何，当时那个年代就是将辐射作为损害因素看待，并没有考虑到剂量的差异。
根据小剂量兴奋效应的观点，任何有毒有害因素，当剂量降低到一定程度，对生物体产生和该有害因素相反的效应，也就是说有毒有害因素剂量降低后不仅不会有害，相反会使生物体产生对抗这种有害因素的能力。这一现象符合生物体适应环境普遍性的进化论观点。因为任何生物都是适应环境的产物，现在生物学研究发现，生物体可以通过表观遗传学途径，随时在不改变基因的前提下，调整基因表达的变化适应环境。那么小剂量辐射效应，从逻辑上看应该存在小剂量兴奋效应。美国目前希望开展的研究，应该只是对这种效应产生的细节和规律进一步研究。
提出这一法案的部分原因是受到日本福岛核电站爆炸事件的影响，美国科学院发起的一项研究，要求能源部制定了一个5年研究计划。但是并没有提供新的研究经费，而且该法案需要经过参议院通过才能成为法律。
Still, supporters of the DOE program, whichhas seen its funding and visibility decline since it was first created in thelate 1990s, are pleased by the House move. “Unfortunately, this program has notbeen a priority at DOE over recent years and has seen systematic de-emphasis,”said Representative Lamar Smith (R–TX), the chair of the House Science, Space,and Technology Committee, in statement before the House’s voice vote on thebill. It will, he said, ensure “the continuance and enhancement of thisimportant research program.”
The nuclear energy industry also welcomedthe move. “Given the pervasiveness of nuclear technologies in our modern world… it just makes sense that we better understand the health effects of low dosesof radiation,” said the Washington, D.C.–based Nuclear Energy Institute, theindustry’s main trade group, in a statement.
The human health effects of low-doseradiation have long been a puzzle. “We know that high doses of radiation causecancer,” says biophysicist David Brenner, director of Columbia University’sCenter for Radiological Research, “but as you go down lower and lower in doseit becomes less and less clear what’s happening.” Detecting slight increases incancer risk, for instance, is difficult. Some scientists have proposed thatthere is a threshold level, below which exposure to radiation is not dangerous,but there is no consensus on whether such a threshold exists, or what the safeexposure level would be, Brenner says.
Radiobiologist William Morgan, director ofradiation biology and biophysics at Pacific Northwest National Laboratory inRichland, Washington, says that researchers are split between multiple theoriesabout low-dose radiation—some say it is good for you, others say it has noeffect, and the rest think it's terrible for you. “Herein lies the confusion inthe low-dose radiation field,” he says.
Congress aimed to help answer such questionsin creating DOE’s low-dose radiation research program in 1999. In particular,researchers and policymakers say solid science is necessary to set appropriateexposure regulations for radiation workers, nuclear power infrastructure, andevacuation plans in the event of a nuclear accident or terrorist attack. Butthe program’s funding has generally declined over the years; low-dose researchsits within a larger DOE funding pool that has gone from a high of some
The 2011 Fukushima accident helped revivelow-dose concerns, however, and in 2013 eight prominent researchers in thefield, including Brenner and Morgan, wrote to White House science adviser JohnHoldren asking for a National Academies report on the issue. They also gavepresentations to the House Energy Committee. The efforts, Brenner says, wereinspired in part by a 2009 road map for systematic low-dose radiation researchproduced by the European Commission and experts from Europe's low-doseradiation research community.
The result was the Low-Dose Radiation ResearchAct of 2015 (H.R. 35), backed by both Democrats and Republicans. It initiallypassed the House last year, but the Senate took no action, meaning thelegislation died in December at the end of the 114th Congress. (The leadingsponsor of that version was Representative Paul Broun [R–GA], who is no longerin office after losing in a U.S. Senate primary.)
Last week, voting on the noncontroversialbill was one of the first bits of business conducted by the newly seated 115thCongress. This time, the lead sponsor was Representative Randy Hultgren (R–IL),a member of the science committee who represents a district that includes DOE’sFermi National Accelerator Laboratory. It again passed on a voice vote.
“While thereis little doubt that there is a threshold above which humans should avoidexposure to radiation, this legislation will ensure that the Department ofEnergy’s Office of Science prioritizes the research necessary to understandwhat that level actually is,” Hultgren said in a speech made on the House floorbefore the vote.
The bill defines a low radiation dose as anexposure of less than 100 millisieverts. For comparison, a person living in theUnited States receives an average annual dose of about 6 millisieverts, half ofwhich comes from natural background sources. A chest x-ray contributes a doseof about 0.1 millisieverts, and a full-body CT scan about 10 millisieverts.
The National Academies report required bythe bill would follow in the footsteps of the 2009 European Commission report.Lawmakers want it to outline the current status of research, address remainingchallenges and scientific goals, and recommend a long-term research agenda. Thebill says the report should consider the cost-benefit effectiveness of theproposed research. DOE would then produce a 5-year research plan that respondsto the findings of the study. The bill does not authorize any additionalfunding for DOE, meaning the agency would have to use existing funds unlesscongressional appropriators say otherwise.
The House bill’s supporters are now lookingfor senators willing to take up the cause. “We are optimistic” about gettingthe bill approved by the Senate, a Hultgren press aide wrote ScienceInsider inan e-mail. The White House has taken no position on the bill.
