二次塑料是由较大的塑料废弃物分解而成的小片塑料。在时间演进之中，历经物理、生物和化学光降解（包括由阳光照射引起的光氧化（英语：Photo-oxidation of polymers）），这些碎片的结构最终降低到肉眼无法察觉到的大小。 [30]这种过程称为破碎化（fragmentation）。[28]微塑料被认为会再进一步降解成尺寸更小的程度，但据报导，目前在海洋中检测到最小微塑料的直径为1.6微米（6.3×10−5英寸）。[31]破碎化持续进行是导致各处普遍均有各式形状微塑料的原因。 [14]据观察，无论在海水或是淡水中，由可生物降解聚合物所形成的微塑料数目，远高于由不可生物降解聚合物所形成的。[32]
航运是造成海洋污染的重要原因。一些统计数据显示在1970年，世界各地的商业船队向海洋环境倾弃超过23,000吨塑料垃圾。在1988年签订的《防止船舶污染国际公约（MARPOL 73/78，附 V）禁止船舶把废弃物倾倒进入海洋环境。美国在1987年通过的海洋塑料污染研究和控制法案（ Marine Plastic Pollution Research and Control Act of 1987）禁止船舶（包括海军舰艇）在海中排放塑料。 [72][73]但航运业仍是塑料污染的主要来源，在1990年代初期即倾弃约650万吨塑料进入海洋。[74][75]研究显示在夏威夷海滩上发现的塑料中约有10%是塑料颗粒。[76]在2012年7月24日，香港经历一场大风暴后，有150吨塑料颗粒和其他塑料原料从附近海岸的船上溢出。据报导，这批来自中国石化公司的塑胶废弃物大量堆积在海滩上。[34]这事件是一起大型泄漏事件，研究人员推测还有更小的事故发生，而进一步加剧海洋微塑料污染。[34]
2008年在华盛顿大学塔科马校区举行名为微塑料海洋垃圾的发生、影响和归宿国际研究研讨会（International Research Workshop on the Occurrence, Effects and Fate of Microplastic Marine Debris），参与者[97]的结论认为微塑料是海洋环境中的一个问题，原因为：
研究人员凯利等人（Kelly et al）在2009年从南极洲东部取样的冰芯中发现有96种微塑料颗粒，属于14种不同的聚合物 。[153]在此之前，南极洲的地表水和沉积物以及北极海冰中都有塑料污染的记录，但这次被认为是首次在南极海冰中发现有塑料污染。这些相对较大的颗粒显示是来自当地的局部污染源。[153]
此外，某些微生物（参见塑料生物圈（英语：plastisphere）#Degradation by microorganisms）已适应食用塑料，一些细菌物种经过基因改造后可食用某些类型的塑料。 [179]微生物除用于降解微塑料外，还可从受污染的样品中捕获其上的生物薄膜基质中的微塑料，以更容易的方式去除此类污染物。[180]还有生物薄膜中的微塑料可利用加工后的“释放”机制，从薄膜扩散而释放，促进微塑料的回收。[181]
随着人们越来越警觉到微塑料对环境的不利影响，相关团体也提倡从各种产品中去除和禁用微塑料。[191]其中一项活动是“打败柔珠”，重点是从个人护理用品中去除塑料。[61]倡议组织Adventurers and Scientists for Conservation开展全球微塑料倡议，目的为收集水样，提供更好的证据给科学家，以了解微塑料在环境中扩散的的状况。[192]UNESCO赞助研究和进行全球评估计划，以处理会跨境影响的微塑料问题。[193]这些环保组织将持续向制造商施压，要求他们从产品中去除塑料，以维持健康的生态系统。[194]
欧盟执行委员会已注意到人们越来越关注微塑料对环境的影响。[204]2018年4月，欧盟委员会的首席科学顾问小组通过欧盟的科学建议机制委托，对微塑料污染的科学证据进行全面审查。[204]审查由欧洲学术机构提名的工作组进行，并于2019年1月交付结果。[205]一份基于SAPEA报告（SCIENCE ADVICE FOR POLICY
BY EUROPEAN ACADEMIES Report）的科学意见向执行委员会提出，委员会将根据资料决定是否应在欧盟层面上提出政策变化，以遏制微塑料污染。 [206]
执行委员会的循环经济行动计划对关键产品（即塑料包装）的回收和减废提出强制性要求。行动计划启动限制在产品中添加微塑料。规定在产品生命周期各阶段捕获更多微塑料的措施。例如该计划将审查目的在减少轮胎和纺织品释放二次微塑料的各项政策。[208]委员会计划更新《城市污水处理指令（英语：Urban Waste Water Treatment Directive）》，以进一步解决微塑料废弃物和其他污染问题，用以保护环境，免受工业和城市废水排放之害。又临时批准对欧盟饮用水指令的修订，以确保定期监测饮用水中的微塑料。如果发现问题，指令要求各国必须提出解决方案。[10]
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