At any given moment, about seven microliters of tears are present in each of our eyes—about a tenth of a drop of water. You might think of them as nothing more than salty water, but it’s more accurate to think if them filtered blood; they deliver oxygen and nutrients to our eyes, removing waste, serving as the first line of defense against pathogens and helping to heal injuries.

Tears also contain traces of the various chemicals originally present in blood, some of which serve as markers of illness—glucose, for example, which can signal diabetes, or enzymes that point to possible liver disease. That’s a primary reason doctors order blood tests. But it also that means that physicians—and maybe, in the near future, you—can look for indicators of illness by looking at your tears.

Research has already shown that markers of many of the most common and devastating diseases, such as cancer, multiple sclerosis, diabetes mellitus, cystic fibrosis and Parkinson’s disease are found in tears. Work is already under way to validate the use of such markers for diagnosing Alzheimer's disease. And recently, a technology named TearExo has been developed for breast cancer screening using tears collected by patients themselves. That could greatly reduce the costs of testing and allow earlier detection of malignancy than mammograms can provide.

Tear testing could also make an impact in the COVID-19 pandemic: current research has focused on developing a test to diagnose SARS-CoV-2 infections, as well as document prior infections, through antibody testing. These assays are typically done with blood—but coronavirus RNA has been detected in tears as well, and antibodies to the virus may also be measured from tears. That means a rapid and cheap tear test could be developed in the near future—and because no needles are involved, this, too, could in principle be self-administered at home.

But the potential advantages of tears as indicators of health go beyond occasional testing. One promising tear-based technology is a smart contact lens that continuously monitors a patient’s biomarkers, significantly improving disease prevention and early detection; it has attracted the attention of Novartis and Google, among other major companies, and is currently under development in research laboratories around the world. One significant step in its development was the first stand-alone contact lens with an integrated battery, in 2019. More recently, a smart contact lens has been successfully developed for continuous glucose monitoring and treatment of diabetic retinopathy. Such a product probably won’t be ready for commercial use for several years. But according to the Centers for Disease Control and Prevention, about 45 million people in the U.S. already wear contact lenses to correct their vision. Switching to a smart version would be simple for them—and of course, you can wear smart lenses even if your vision is perfect,

As with detecting diseases, we can use mouse models to find tear markers for conditions ranging from environmental pollution to chronic stress to drug abuse. In 2019, I worked on developing a technique allowing the first-ever metabolite marker screening from mouse tears. This was challenge because a mouse’s tears have less than 10 percent of the volume of human tears. I was able to overcome this challenge by using thin threads to absorb very small amounts of liquid from the surface of the eye, similar to how a paper towel absorbs water.

Interestingly, in 2018, NASA proposed to use tear markers to monitor astronauts' health in space. This would allow us to better understand the effects on human health of long-term exposure to space environmental factors such as radiation and weightlessness. Tear markers from veteran astronauts and new astronauts without space exposure would be used to establish a health database, which might be one of the first stepping-stones toward making space colonization a living reality.

This is just the tip of the iceberg when it comes to the potential in tears beyond crying. Tears can provide an inexpensive, rapid and easy-to-use alternative to monitor health. Given the unprecedented advantage in tear technology to externally and continuously monitor health, and to be used even by the healthy, such technology will almost certainly lead more effective prevention of diseases—rather than having to deal with the difficulties of treating them.