虽然大多数美国人说他们在公共场合外出时戴着面具，一些州已经采取了更严格的措施来确保这种情况在2010年的冠状病毒pandemic.
据倡导更多戴面具的志愿者组织“马斯卡索”称，在50个州中，有46个州要求在全州范围内戴面具，或者在某些地方有某种要求。
最严格的面具要求是，当不可能与社会保持距离时，市民必须一直在外面戴着面具，重点是杂货店和公共场所运输。十八个州，和波多黎各和华盛顿特区都属于这一类。
更宽松的要求要求只有开放企业的员工——而不是顾客——在室内戴口罩。
四个州——爱荷华州、蒙大拿州、威斯康星州、南达科他州——根本没有口罩要求。
哪些州需要口罩?
哪些州需要口罩?广播公司新闻
研究证明了口罩的功效。
一个研究发表在同行评议的医学杂志《柳叶刀》上的研究发现，戴口罩可以将传播风险从17%降低到3%。
人们发现多层口罩比单层口罩能更好地保护公众。
这项研究着眼于先前对COVID-19和相关冠状病毒如非典和中东呼吸综合征传播的研究。
2020年6月25日，在威斯康辛州的两河市，人们在内绍塔海滩上看热闹
2020年6月25日，在威斯康辛州的两河市，人们在内绍塔海滩上看热闹加里·克莱因/今日美国网络
在美国各地爆发反对警察暴行的抗议活动后，面具的有效性也得到了证实
尽管担心抗议活动可能会导致病例增加，但一些州的卫生官员和州长称情况并非如此，并指出许多抗议者戴着口罩。
在明尼苏达州，卫生部的一名官员告诉美国广播公司新闻，他将低感染率归因于“许多或大多数抗议者戴着面具，事件发生在室外，人们通常能够保持6英尺的距离，任何接触的持续时间相对较短。”
马萨诸塞州州长查理·贝克(Charlie Baker)同样表示，在接受检测的抗议者中，COVID-19的阳性率与该州的每日数据一致——换句话说，与非抗议者的阳性率相同。
2020年6月22日，星期一，纽约布鲁明戴尔公司的商店橱窗里展示着戴着防护面具的人体模型。
2020年6月22日，星期一，纽约布鲁明戴尔公司的商店橱窗里展示着戴着防护面具的人体模型。吉纳月亮/彭博通过盖蒂图片
“参加这些示威的绝大多数人都戴着这样或那样的面具或面罩。在许多情况下，他们都在移动，我认为这产生了很大的不同，当然他们都发生在室外，”贝克说。
随着大流行的继续，美国大多数戴口罩的人都增加了。
一项新的美国广播公司新闻/益普索民意调查发现，在上周离家的美国人中，将近90%的人说他们戴了口罩或面罩，相比之下，只有1%的人说他们没有戴。
当这个问题最后一次在4月中旬被问到时，只有69%的美国人说他们在公共场合戴着面具，这一比例高于4月10日公布的一项调查中的55%。
Which US states require masks and which 4 don't at all?
While themajority of Americanssay they are wearing masks when they go out in public, some states have taken more stringent measures to ensure that's the case during thecoronavirus pandemic.
Of the 50 states, 46 either require masks to be worn statewide or have some sort of requirement in certain locales, according to Masks4All, a volunteer organization that advocates for more mask-wearing.
The most strict mask requirement involves citizens wearing a mask outside at all times when social distancing isn't possible, with an emphasis on grocery stores and publictransportation. Eighteen states, andPuerto Ricoand Washington, D.C., fall under this category.
More lenient requirements require that only employees — not patrons—of businesses that are open wear masks indoors.
Four states — Iowa, Montana, Wisconsin, South Dakota — have no mask requirements at all.
Studies have proved the efficacy of masks.
Onestudypublished in The Lancet, a peer-reviewed medical journal, found that wearing a mask may drop the risk of transmission from 17% to 3%.
Multilayer masks were found to have offered better protection than single-layer masks for the general public.
The study looked at prior research on transmission of COVID-19 and related coronaviruses like SARS and MERS.
Masks' effectiveness was also proven after protests against police brutality erupted across the U.S.
Despite a concern that the protests may lead to a rise in cases, health officials and governors in some states have said that wasn't the case, noting that many protesters were wearing masks.
In Minnesota, an official for the Department of Health told ABC News that he attributes the low infection rate to “the fact that many or most protesters were wearing masks, the events were outside, people were often able to maintain a six-foot distance, and any exposures were of relatively shorter duration.”
Massachusetts Gov. Charlie Baker similarly said that the positive rate for COVID-19 among protesters who got tested was consistent with the daily state figure—in other words, the same as for non-protestors.
The "vast majority of the folks who participated in those demonstrations were wearing masks or face coverings in one kind or another. In many cases they were moving, which I think made a big difference, and of course they all took place outside," Baker said.
The majority of people in the U.S. wearing masks has increased as the pandemic has gone on.
Nearly nine in 10 Americans who left their home in the last week said they wore a face mask or face covering, compared with only 1% who said they did not, a new ABC News/Ipsos poll found.
When the question was last asked in mid-April, only 69% of Americans said they wore a mask in public, which was up from 55% in a survey released on April 10.
