In classrooms across the United States, a simple intervention is taking place. Without any high-tech devices or complicated procedures, just a tiny brush and a swipe of clear liquid.

This simple intervention—fluoride varnish applied at school—is proving to be one of the most effective ways to combat childhood cavities. And now, thanks to a sweeping review of evidence, experts say it may be time to treat school-based fluoride programs not as an option, but as essential public health infrastructure.

A simple shield against a widespread problem

Dental cavities remain one of the most common chronic diseases in children, despite being largely preventable. According to national data, 56.8% of adolescents aged 12 to 19 have had at least one cavity. Untreated decay can lead to pain, missed school days, difficulty eating and speaking, and even long-term academic consequences.

Yet, far too many children (especially those from low-income families or communities of color) never receive basic dental care. In fact, less than 18% of low-income children in the U.S. received a fluoride treatment during a dental visit in 2013–2014. That figure hasn’t improved much since.

Fluoride varnish offers a way around those barriers. The treatment, recommended by the American Dental Association and the U.S. Preventive Services Task Force, is quick, safe, and cost-effective. It can be applied in classrooms, gymnasiums, or even cafeterias by dentists, hygienists, nurses, or trained lay workers.

Fluoride varnish is a highly concentrated form of fluoride applied to the teeth as a protective coating to help prevent tooth decay. It involves brushing a sticky, resin-based liquid onto the teeth, where it hardens on contact with saliva. The fluoride strengthens tooth enamel, making it more resistant to acid attacks from bacteria and sugars in the mouth.

The largest-ever systematic review of school fluoride varnish delivery programs, published in the American Journal of Preventive Medicine, analyzed 31 studies involving more than 60,000 students in preschool through high school. It found that these programs reduce cavities in permanent teeth by 32%, and by 25% in baby teeth. Just as important, they prevent small cavities from worsening, cutting progression by 10% on average.

“This is very impactful,” says Dr. Anthony Leonetti, who was not involved with the study. “It’s one of the most impactful interventions you can do at this stage.”

This can easily be scaled

The good thing about the “fluoride varnish” is that it doesn’t require special dental devices and can be applied quickly in various settings. Critically, it’s effective in both urban and rural areas, with or without fluoridated water.

Schools are also an excellent starting point for it, because that’s where children already are. They’re also uniquely positioned to reach those most likely to fall through the cracks of the healthcare system. Many programs target communities where cavity risk is highest—places where dental offices may be scarce, transportation is a hurdle, or Medicaid coverage is spotty. In many cases, these programs do more than just apply varnish. They provide oral health education, distribute toothbrushes and toothpaste, and refer children for needed dental care.

The need for school-based fluoride programs is growing more urgent as some communities reconsider one of the longest-standing tools in public health. Across the U.S., a growing number of local governments are moving to reduce or eliminate fluoride from public water supplies—a reversal that could have widespread consequences for dental health.

“In addition, we believe that since there is a growing movement in the U.S. to remove water fluoridation, other ways of protecting teeth with fluoride, such as toothpaste and varnish, will become more important. About three-quarters of the U.S. population using public water systems has been receiving fluoridated water at levels designed to strengthen enamel and prevent cavities. They will be at higher risk for cavities if fluoride is removed from their drinking water,” write the authors in an article on The Conversation.

Not only do these programs reduce cavities—they also shrink disparities. In one Swedish study, children in lower-income areas benefited far more than those in wealthier neighborhoods. Another large study in Scotland confirmed that the effects of fluoride varnish were strongest in areas of high social deprivation.

Despite their success, school fluoride varnish programs remain underused. Roughly 30% of U.S. states report having no such programs at all. Even where they exist, policy obstacles make them difficult to sustain or scale.

Fluoride varnish is no silver bullet. It doesn’t replace regular dental check-ups, sealants, or cleanings. But when so many children never make it to the dentist, this small, simple act in a school hallway can be transformative.