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阅读下面科普短文.

日常生活中,我们常饮用桶装水.网上盛传“千沸水”(指反复多次烧开的水)中含有大量亚硝酸盐,摄入一定量可导致中毒;此外,亚硝酸盐在人体内还能形成强致癌性物质﹣亚硝胺,饮用会致癌.这是真的吗?

水中亚硝酸盐主要于微生物的还原作用,适宜的温度会使水中的微生物增多,微生物释放出的还原酶可将水中的硝酸盐还原成亚硝酸盐.

童贵忠、董智军等人对微生物检测的实验:

取15种桶装水与饮水机装配好,分别测使用过程(动态)中不同时段水桶内的水样.取13种桶装水(不与饮水机装配)静态放置进行检测.检测结果如下表:

样品类别

数量

微生物超标数

微生物超标率/%

动态实验样品(72小时)

15

14

93.0

静态实验样品(1个月)

13

2

15.4

解释:使用过程中,随桶中水量的减少,空气进入水桶中也会带入许多空气中的微生物.加热时,桶内水温会达到微生物大量繁殖的适宜温度.

加热次数对亚硝酸盐含量是否有影响呢?我国在《GB 19298﹣2003瓶(桶)装饮用水卫生标准》中对亚硝酸盐(以NO2﹣计)的含量规定为小于5 μg/L.

邹伶俐、张雪娇等人的实验:

取新鲜的桶装水放置在饮水机上,测定36h内反复加热(共计181次)的水样,亚硝酸盐含量(以NO2﹣计)检测结果如下表:

加热时间/h

0

1

3

6

12

24

36

含量/μg/L

0.67

0.73

1.12

1.23

1.81

2.44

3.53

取检验合格的14桶桶装水分两组(每组7份)静态放置,进行存放环境检测,亚硝酸盐含量(以NO2﹣计)检测结果如图.

人在日常生活中主要摄取亚硝酸盐的途径是蔬菜,通过人体唾液的转化,间接进入人体的亚硝酸盐含量最高在300mg/kg,人并没有因为食用蔬菜而导致亚硝酸盐中毒,由此可知“千沸水”中的亚硝酸盐不足以使人中毒.没有明确证据证明亚硝酸盐能致癌,在正确使用饮水机的前提下,“千沸水”不会致癌.反复煮沸的水,只是活性降低了,不易被人体细胞吸收.

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