新托福阅读训练：来自宇宙的辐射

会计考友

癌症是令所有人闻之色变的可怕疾病，但很多人都不知道来自宇宙的辐射会是导致癌症产生的原因之一。  
　　Did you know that cosmic radiation is up to one-hundred times greater at cruising altitude than on the ground? Learn more on this Moment of Science.

　　Did you know that cosmic radiation is up to one-hundred times greater at cruising altitude(巡航高度) than on the ground? If you need another reason to dislike flying, this Moment of Science is for you.
5 W5 k. j- O: V. ]/ s" [/ F5 c
　　Like all radiation, cosmic radiation is made of electrically charged particles(带电粒子) such as protons(质子) and ions(离子).These particles drift to earth from space: The sun is a major source, especially during solar flares(太阳耀斑). Just like more familiar forms of radiation, for example Radon and X-rays, high dose cosmic radiation may be associated with health problems such as cancer.

　　Luckily, most cosmic radiation is screened out (遮挡)by our atmosphere before it ever reaches the ground. At sea level, the atmosphere is thick and screens out a lot, but at high altitudes, such as the cruising altitudes of many commercial airplanes, we are less protected.

% |' d  u3 g1 t4 X( _/ F( J　　Is this a reason to avoid flying? Well, not really. While radiation levels are greater in aircraft than on the ground, your overall dose is still quite small. You would need to spend thousands of hours in the air to suffer even a slight increase in your risk of health problems. But, cosmic radiation might be a concern for flight crews, who fly day after day over many years. For most of us, when it comes to cosmic radiation we are on pretty safe ground.