PM2.5是人类健康杀手

研究表明,不同的空气污染物对人体健康有不同的影响,而可吸入颗粒尤其是颗粒物细粒子PM2.5是各种空气污染物中对人类健康影响最大的因素。

科学家们对PM2.5的健康危害性的研究正在日益深入。由于PM2.5非常细小,所以具有一些特点,使得它对健康的影响不容忽视:一是非常容易穿过鼻腔的阻挡,并通过气管进入人的肺泡,其中小于100纳米(1纳米等于千分之一微米)的极细粒子甚至能穿过肺部进入血液,并影响到其他器官,包括大脑;二是比表面积(即单位质量颗粒物的总表面积,颗粒越小,它的比表面积就越大)大,能吸附大量的有毒、有害物质,如强致癌性的多环芳烃(一类分子中含有2个以上苯环的化合物,主要由燃料的不完全燃烧而产生)和重金属等,且各种物质在颗粒物表面会发生非常复杂的化学反应;三是在大气中的滞留时间长、输送距离远,因此影响的范围广。

科学家主要通过两种方法研究PM2.5对健康的影响。一个是通过流行病学调查,统计分析PM2.5浓度与死亡率及各种疾病发病率之间的相关性,从而判断它们之间的因果关系;另一个则是通过病理实验,研究PM2.5引起各种疾病的机理。

根据世界卫生组织的评估,可吸入颗粒物是各种空气污染中对人类健康影响最大的因素。在欧盟,PM2.5使得每个欧洲人的预期寿命平均缩短了8.6个月。流行病学研究表明,颗粒物的短期或长期暴露都会对人体健康产生不利影响。颗粒物对健康的影响是多方面的。,但主要影响呼吸系统和心血管系统。颗粒物对人体健康的影响还有一个特点,那就是不存在一个最低的浓度阈值,即低于该浓度的暴露不会引起健康危害。例如,在美国和西欧国家,产生健康危害效应的PM2.5浓度估计只比环境背景高 3~5 微克/立方米。因此从理论上来说,大气颗粒物的浓度越低越好。世界卫生组织制定的空气质量标准,就是基于流行病学的研究结果。

病理学的研究发现,氧化应激及炎症反应是大气污染影响人体健康的关键机制。当人们吸入PM2.5时,呼吸系统会首先受到颗粒物中氧化性化学物质如自由基的刺激,产生炎症反应。进入血液的超细颗粒物会造成血管内皮细胞损伤,也很容易引发炎症反应。随后,炎症反应可能会扩展到全身。在炎症反应中,人体内的血小板增多、活化、凝聚,导致血流变缓,血液的凝固性增加。因此,经常暴露在PM2.5污染环境中,就可能出现动脉粥样硬化,形成血栓,最终导致心肌梗死。PM2.5还会刺激肺部神经,引起神经反射,进而影响到自主神经系统。自主神经系统是不受意识控制的末梢神经系统,又称植物性神经系统,包括交感神经系和副交感神经系,控制着很多重要的生理功能,如心脏搏动、呼吸、血压、消化和新陈代谢等,当它的功能发生紊乱,就会导致心律不齐等症状。PM2.5通过这些机制的共同作用,引起心血管系统的病变。另外,PM2.5中的多环芳烃等物质进入人体后,经过代谢会变成具有极强的致突变性的环氧化物,使人增加患上癌症的风险。

PM2.5来源与化学组分很复杂,毒理学研究显示,不同化学组分的毒性机制和效应有很大差异。除了呼吸系统和心血管系统以外,有证据显示PM2.5还会影响免疫系统、神经系统、生殖系统和遗传等。PM2.5影响人体健康的机制非常复杂,还没有完全研究清楚。

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