手机使用率居高不下，为何脑癌发病率却在下降？

During the 1980s, just as Americans began pumping low-frequency radiation through their skulls with cell phones, brain cancer rates in the U.S. slowly increased. At the beginning of the decade, doctors delivered the devastating diagnosis of brain cancer to 63 out of every 1 million Americans every year; by 1990 that number had risen to 70 per million. And that's when cell phone usage really took off.

Yet while the link between phones and tumors may have seemed certain to grow, a strange thing happened. Beginning in 1991 the rate of brain cancer incidence reversed course and began to slowly fall. By 2008, the last year for which the National Cancer Institute has data, 65 out of every 1 million Americans got a brain cancer diagnosis annually.

During those same two decades, the radiation beamed by phones into American brains increased about 500-fold, in proportion to the wild growth in cell phone use. There are roughly 60 times more cell phones in the U.S. than there were in 1990, and each one is used for an average of about 20 minutes per day, up from just a minute or two in the industry's expensive early days. The combined effect of more customers each talking more has been a stunning increase in total talk time.

Combining data from the National Cancer Institute and the cellular industry's main trade group yields this intriguing graph:

布朗大学（Brown University）的传染病科教授戴维•萨维茨曾进行过这类研究，他警告说，虽然从图中能很清晰地看到手机辐射量与脑癌发病率之间没有关系，但并不能完全证明手机是安全的。最令人担忧的是，使用手机和最终罹患脑癌之间可能存在漫长的时间间隔。

不过塞维茨同时也认为，（低频）辐射量大幅增加不会导致脑癌患者总人数上升这一发现是令人信服的。即便辐射暴露与罹患脑癌之间的平均时间间隔长达30年，但现实中必然会有在20年后就患病的特例者，这些人肯定会体现在目前的数据中。（情形与人们早期受到石棉等物质辐射，很长时间后才出现症状；类似。）虽然1990年时的手机用户总数非常低，但也有足足500万人。在2008年时哪怕这些早期使用者中只有很少一部分人染上因手机辐射导致的脑癌，那么当年的脑癌发病率也会出现大幅上升。

曾担任过《美国流行病学杂志》（American Journal of Epidemiology）编辑一职的萨维茨表示：“真正的问题是，病情什么时候开始爆发？假如没有迹象表明问题是何时开始发生的，那么相对于其它我们研究过的诱因，这次的情况可能将是史无前例的。”

上周，世界卫生组织（World Health Organization）发布了一项研究报告，认定手机“可能致癌”，此结论主要基于一项研究发现：重度手机用户罹患神经胶质瘤的风险增加了40%。据报道，这些用户在10年里平均每天使用手机通话的时间超过30分钟。对于典型的美国电话电报公司用户（AT&T）而言，这个消息有点可怕。今年第一季度，他们的平均每天通话时间达到了21分钟。而对于MetroPCS和Leap Wireless的用户来说，这个消息简直就是个噩耗。MetroPCS和Leap Wireless是专门从事替代固话服务的等廉价服务商。目前，Leap的语音用户平均每天通话时间已经达到了惊人的50分钟。

不过，至少到目前为止，所有重度用户脑癌的整体发病率并没有上升。

手机到底会不会导致脑癌？要得到这个问题的最终答案，我们还得等上几十年时间，等到人们所接受的辐射量真正稳定在高水平，而脑癌发病率的趋势出现明显变化，要么掉头上升，要么继续下降。目前，辐射量已经开始趋于稳定。在经历了30多年的高速发展后，美国用户的手机通话时长在2010年时已经稳定在了每年2.7万亿分钟的水平上。

译者：项航

The chart's clear disconnect between radiation dose and cancer rates does not conclusively prove cell phone are safe, cautions David Savitz, a Brown University epidemiology professor who has studied the issue. Most worrisome, it's possible there is simply a huge delay between a person using a cell phone and that exposure causing a brain tumor.

Yet Savitz also finds the utter lack of an increase in total brain cancer following the massive increase in (low-level) radiation very reassuring. Even if the average lag between exposure and symptoms was 30 years, there would almost certainly be outliers who develop symptoms after just 20 years -- outliers who would be showing up in the data by now. (That's what's happened after early exposures to things like asbestos.) Total cell phone use may have been far lower in 1990, but there were already 5 million subscribers. It wouldn't take many of those early adopters coming down with cell phone-caused brain cancer to bump up the cancer rates in 2008.

"The real question is: When would we see the beginning of the epidemic?" says Savitz, the former editor of the American Journal of Epidemiology. "It would be unprecedented relative to other agents we've studied to have no evidence of the beginning of the problem."

The World Health Organization study released last week classified cell phones as "possibly carcinogenic to humans," based largely on a study that found a 40% increased risk for a cancer called gliomas among heavy cell phone users. Those users reported averaging 30 minutes per day talking on their phones over a 10‐year period. That's somewhat scary news for the typical AT&T (T) customer, who averaged 21 minutes a day in the first quarter of this year. And it's really scary for customers of low-cost providers that specialize in replacing landline service such as MetroPCS (PCS) or Leap Wireless (LEAP). Customers of Leap's Cricket service average a whopping 50 minutes per day.

And yet, so far at least, all that talking hasn't jacked up the overall incidence of brain cancer.

The ultimate answer to the question of whether cell phones cause brain cancer will only be definitively resolved after a few more decades in which the population's radiation dose stabilizes at a high level and brain cancer rates either jump or continue to decline. That stabilizing has already happened. After three decades of stunning growth, time spent talking on cell phones in the U.S. leveled off in 2010 at 2.7 trillion minutes a year.

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