When we hear the word radiation, it's tempting to picture huge explosions and frightening mutations, but that's not the full story. Radiation also applies to rainbows and a doctor examining an x-ray. So what is radiation really, and how much should we worry about its effects? The answer begins with understanding that the word radiation describes two very different scientific phenomena: electromagnetic radiation and nuclear radiation. Electromagnetic radiation is pure energy consisting of interacting electrical and magnetic waves oscillating through space. As these waves oscillate faster, they scale up in energy. At the lower end of the spectrum, there's radio, infrared, and visible light. At the higher end are ultraviolet, X-ray, and gamma rays. Modern society is shaped by sending and detecting electromagnetic radiation. We might download an email to our phone via radio waves to open an image of an X-ray print, which we can see because our screen emits visible light. Nuclear radiation, on the other hand, originates in the atomic nucleus, where protons repel each other due to their mutually positive charges. A phenomenon known as the strong nuclear force struggles to overcome this repulsion and keep the nucleus intact. However, some combinations of protons and neutrons, known as isotopes, remain unstable, or radioactive. They will randomly eject matter and/or energy, known as nuclear radiation, to achieve greater stability. Nuclear radiation comes from natural sources, like radon, a gas which seeps up from the ground. We also refine naturally occurring radioactive ores to fuel nuclear power plants. Even bananas contain trace amounts of a radioactive potassium isotope. So if we live in a world of radiation, how can we escape its dangerous effects? To start, not all radiation is hazardous. Radiation becomes risky when it rips atoms' electrons away upon impact, a process that can damage DNA. This is known as ionizing radiation because an atom that has lost or gained electrons is called an ion. All nuclear radiation is ionizing, while only the highest energy electromagnetic radiation is. That includes gamma rays, X-rays, and the high-energy end of ultraviolet. That's why as an extra precaution during X-rays, doctors shield body parts they don't need to examine, and why beach-goers use sunscreen. In comparison, cell phones and microwaves operate at the lower end of the spectrum, so there is no risk of ionizing radiation from their use. The biggest health risk occurs when lots of ionizing radiation hits us in a short time period, also known as an acute exposure. Acute exposures overwhelm the body's natural ability to repair the damage. This can trigger cancers, cellular dysfunction, and potentially even death. Fortunately, acute exposures are rare, but we are exposed daily to lower levels of ionizing radiation from both natural and man-made sources. Scientists have a harder time quantifying these risks. Your body often repairs damage from small amounts ionizing radiation, and if it can't, the results of damage may not manifest for a decade or more. One way scientists compare ionizing radiation exposure is a unit called the sievert. An acute exposure to one sievert will probably cause nausea within hours, and four sieverts could be fatal. However, our normal daily exposures are far lower. The average person receives 6.2 millisieverts of radiation from all sources annually, around a third due to radon. At only five microsieverts each, you'd need to get more than 1200 dental X-rays to rack up your annual dosage. And remember that banana? If you could absorb all the banana's radiation, you'd need around 170 a day to hit your annual dosage. We live in a world of radiation. However, much of that radiation is non-ionizing. For the remainder that is ionizing, our exposures are usually low, and choices like getting your home tested for radon and wearing sunscreen can help reduce the associated health risks. Marie Curie, one of the early radiation pioneers, summed up the challenge as follows: "Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less."
聽到輻射這個詞的時候 很容易聯想到大爆炸的畫面 跟可怕的基因突變 其實不全然是那一回事 因為彩虹 和醫生檢查用的 X 光也算輻射 那輻射到底是什麼 對其影響我們又該多憂心呢? 答案要先從瞭解「輻射」這個詞開始 是描述兩種截然不同的科學現象 那就是電磁輻射 和核輻射 「電磁輻射」是純能量 由電波與磁波通過空間 相互作用振盪所構成 隨著電磁波振盪速度變快 能量隨之變大 光譜低頻端有無線電波 紅外線 和可見光 高頻端則有紫外線 X 射線 和伽瑪射線 現代社會是藉由 發送和檢測電磁輻射成形的 我們能透過無線電波 把郵件下載到手機 來開啟 X 光片的影像 影像之所以可見 是因為螢幕發射出可見光 另外一種就是「核輻射」 它來自原子核,核中的質子 因為都帶正電荷而相斥 一種現象稱為「強核力」 會努力克服質子相斥 同時維持原子核的穩定 然而,某些質子與中子的組合 也就是「同位素」 依然不穩定 亦即具有放射性 會隨機放射物質 和/或 能量 ─ 稱為「核輻射」 ─ 以使自己達到較大穩定度 核輻射來自大自然,例如氡 是一種由地下向上滲出的氣體 我們還精煉天然的放射性礦石 提供燃料給核電廠 甚至連香蕉裡都含有 微量的鉀放射性同位素 因此,我們要是生活在輻射世界裡 該怎麼避免其危險與影響呢? 首先,非所有輻射都具危害 當輻射撞擊出原子中的電子時 才會有危險 這過程會對 DNA 造成傷害 這種輻射就叫「游離輻射」 因為原子失去或得到電子 就稱為離子 所有核輻射都是游離輻射 而電磁輻射只有最高能量的 才是游離輻射 包括 γ 射線 X 射線 和高能量端的紫外線 這就是為何照 X 光要做額外防護措施 醫生會遮住不需檢查的身體部位 以及為何沙灘客要擦防曬油 相較之下 行動電話和微波爐放射低頻光譜 所以使用上沒有游離輻射的危險 對健康造成最大威脅 是大量游離輻射 在短時間內擊中人體時 也稱為「急性曝露」 急性曝露把人體 自己修復能力破壞殆盡 會誘發癌症 細胞功能障礙 甚至可能死亡 所幸,急性曝露相當罕見 但是我們每天都曝露在 低劑量游離輻射下 ─ 來自自然界和人造物品 科學家很難將這些風險量化 人體經常要修復 由少量游離輻射造成的損害 若修復不了 損害結果可能十年或更久 都不會顯現出來 科學家用來 計算游離輻射曝露量的方法 是稱為「西弗」的單位 急性曝露在一西弗下 可能在數小時內出現噁心症狀 四西弗可能會致命 然而,我們日常曝露量低得多 一般人每年從各種來源 接觸的輻射量是 6.2 毫西弗 約三分之一是來自氡 以每次照射只有 5 微西弗計 必須要超過 1200 次的口腔 X 光照射 才能達到一年的劑量 還記得香蕉嗎? 人體若能將香蕉的輻射劑量完全吸收 則必須一天吃 170 根香蕉 才能達到整年劑量 我們雖生活在充滿輻射的世界 然而,大部分輻射都是 非游離輻射 就其餘是游離輻射而言 我們的曝露量通常很低 有些選擇 像檢測家中氡氣 外出擦防曬 都能幫助減少危害健康的風險 早期輻射先驅之一 瑪麗·居禮(居禮夫人) 對挑戰作了以下結論: 「生活中沒什麼可怕的事,只有需要被瞭解的事。 現應當多一些瞭解,才能少一分擔心受怕。」