As the coronavirus’s delta variant spreads across the United States, Americans are again debating whether schools should mandate that students and employees wear masks to slow transmission. The Centers for Disease Control and Prevention has recommended that all unvaccinated students wear masks inside schools, and the American Academy of Pediatrics recommends that everyone wear masks inside schools, regardless of vaccination status.

Will school districts heed public health guidance and adopt mask mandates? The answer may depend on how strong the local teachers union is, our research finds. Here’s what we know.

How we did our research

At the start of the 2020-2021 school year, 38 states had statewide public mask mandates that included schools. The remaining 12 states had no such mandate, instead leaving the mask decision up to local school districts. This fall, the situation is largely reversed: 13 states have statewide mask mandates; 31 are leaving it up to school districts; and six have banned local districts from requiring masks inside schools

In a study we recently published with Simeon Kimmel and Atheendar Venkataramani, we examined mask mandates last year in Iowa. We studied Iowa for two reasons. First, Iowa was one of the 12 states that left the decision up to school districts. Second, Iowa’s right-to-work law historically made it difficult to unionize teachers and generated much higher district-level variation in unionization than is seen in states with more favorable labor legislation.

Without a statewide mandate, only 60 percent of Iowa school districts required masks. So what distinguished those districts with mask mandates from those without?

To find out, we gathered district-level data on the teachers’ unionization rate, or the percentage of teachers who were members of the Iowa State Education Association, the state’s teachers union. We also gathered data on other factors that may explain the adoption of mask mandates, such as county-level coronavirus case rates, the percentage of voters who voted for Donald Trump in the 2016 presidential election, district-level student enrollment, racial demographics, urban/rural classification, and other socioeconomic factors. We then used a statistical technique called “regression analysis” to see which of these factors best explained the local adoption of mask mandates.

Last year, stronger unions meant mask mandates were more likely

Many Iowa districts reopened last year without mask mandates. In districts where fewer than 30 percent of teachers were unionized, only 27 percent of schools adopted mask mandates. We found that a 10-point increase in the teachers’ unionization rate was associated with a four-point increase in the probability of a school district adopting a mask mandate. This means that the average teachers union was associated with a 33 percent relative increase in the probability of school districts adopting a mask mandate. That was true even when controlling for local coronavirus case rates, support for Trump, and the other factors mentioned above.

Iowa’s teachers unions pushed for mask mandates via local advocacy and partnership with school boards — strategies that teachers unions throughout the country are pursuing. For example, the Iowa State Education Association developed a “safety checklist,” including mask mandates, that it encouraged local school districts to adopt. Union locals shared these recommended policies with elected officials and the public at open school board meetings and worked with school administrators to have union members placed on the district-level committees that planned the health and safety measures implemented before schools reopened in fall 2020.

Consider Mary Heeringa, a librarian and teacher who was president of a local union affiliated with the Iowa State Education Association. After she learned her governor would not adopt a statewide mask mandate, she read the union’s safety checklist aloud at a public school board meeting. Along with other union members, she attended various committees responsible for a safe return in the fall and helped persuade them that masks were necessary. Her district ultimately adopted a mask mandate, an outcome she attributed to her union’s history of working closely with the school board.

This year, some teachers’ unions won’t have a voice

This school year, Iowa districts are unlikely to be allowed to implement mask mandates, regardless of union efforts. Earlier this year, Gov. Kim Reynolds (R) signed a bill that prohibits local districts from requiring masks inside schools. Five other states, all with Republican governors, have imposed similar bans on local mask mandates.

There are, however, 31 states that will allow local districts to decide their own mask policies. And there is growing evidence that teachers unions outside of Iowa will fight for and secure mask mandates for this coming school year.

Massachusetts, for example, ended its statewide mask mandate in May. However, Boston’s schools just announced that everyone will be required to wear a mask indoors when schools reopen. The Boston Teachers Union supported the decision, arguing that “these steps are necessary to protect the health and safety of our students.” Similar local school mask mandates have been announced in Columbus, Ohio, and Kansas City, Mo., two places where the delta variant is spreading rapidly.

These mask mandates, which unions help secure, protect more than just teachers. Schools that required masks last year had a 37 percent lower incidence of coronavirus cases than those that did not. And that reduction within schools also slowed community spread.

Teachers unions weren’t alone. In another study, we found that unionized nursing homes in New York were more likely to have access to N95 masks and had resident mortality rates that were 30 percent lower than nonunionized nursing homes — which in turn helps protect the wider community. After more than a year of debating how the country can protect essential workers, evidence suggests that unionized essential workers can help protect the country.

Adam Dean (@adamdean34) is a professor of political science at George Washington University whose research focuses on the political economy of international trade, labor politics and the socioeconomic determinants of public health.

Jamie McCallum (@jamiekmccallum) is a professor of sociology at Middlebury College whose research focuses on labor movements, unions and workplace power relations. McCallum is writing a book on labor issues during the pandemic.