If companies like Johnson & Johnson, IBM, and PepsiCo get their way, millions of American workers will see their private health information become public knowledge. That may not be good for them.

The concept isn’t new, but it got a big push last month at the World Economic Forum in Davos, Switzerland. The Vitality Group, a company that designs corporate health promotion programs, unveiled a proposal for publicly held companies to start reporting employees’ health information to investors and other corporate stakeholders. Body Mass Index (BMI), smoking status, anxiety and stress levels, and other health-related data would be anonymized, aggregated, and analyzed to give investors a different kind of snapshot of the company’s health.

Many experts believe the ultimate goal here is to slow the rise in US health-care costs by rewarding employees who meet health benchmarks and punishing those who don’t. To some extent that’s already happening. Under the US’s Affordable Care Act, passed in 2010, employers can penalize workers who don’t sign up for screenings and wellness programs, charging them up to 30% more for health coverage than employees who do.

That might or might not bring down companies’—and the country’s—health-care bills. Either way, it comes with some worrying possible side-effects.

One is that employees could be linked to their health data—a kind of outing of medical conditions that people might not want to reveal, especially to employers. “Aggregation is supposed to do the work of privacy here,” explains Jessica L. Roberts, associate professor of law and director of the Health Law & Policy Institute at the University of Houston Law Center. That might hold true for companies with, say, 50,000 employees. But people at smaller firms would be much more vulnerable to exposure, even with their data anonymized.

Even the Obesity Action Coalition, a Florida-based national nonprofit supported by the bariatric industries—providers of goods and services to the obese—thinks this move could be problematic. As a spokesman for the OAC told STAT News , “It does inadvertently put a target on employees’ backs.”

And maybe not so inadvertently. Some companies, like Wisconsin-based manufacturer Flambeau, have refused health coverage to workers who don’t fill out health questionnaires and/or undergo extensive health testing. Others, like Citizens Medical Center in Victoria, Texas, have declined to hire people considered obese according to the BMI chart. (Citizens, which instituted its hiring policy in 2011, reversed it a year later after a deluge of bad publicity.)

The federal Americans with Disabilities Act makes it illegal for companies to hire or fire people on the basis of disability or the disability of a family member. The federal Equal Employment Opportunity Commission has challenged (paywall) some of the obligatory wellness programs in court, with varying results. But, Roberts points out, protections like the law on disabilities are likely not enough. “There is no federal statute forbidding employers from firing people who are unhealthy or risky,” she says.

Nor from refusing to hire them in the first place. Employers in 21 states, including Texas, Florida, and Pennsylvania, can already refuse to hire smokers, even if they don’t use nicotine on the job. And companies who don’t want to pay health-care costs for workers they perceive as unhealthy could rewrite hiring policies to cherry-pick “healthier” employees.

That’s a worry, says Roberts. It could lead to nannyism if companies decide to micromanage workers’ health to make themselves look more profitable. In any case, she says, “It’s unclear whether the health of the workforce is a good proxy for the quality of the investment.” For instance, people classified as unhealthy because they’re more sedentary might actually be more productive workers. Another example: A new study published in the International Journal of Obesity found that using BMI as a health measure misclassifies some 75 million Americans.

“We live in an era of big data,” acknowledges Roberts. “We have access to more information than ever before.” It’s just not clear in this case how useful or reliable that information might be.