Medical devices are gathering more and more data from their users, whether it’s their heart rates, sleep patterns or the number of steps taken in a day. Insurers and medical device makers say such data can be used to vastly improve health care.

But the data that’s generated can also be used in ways that patients don’t necessarily expect. It can be packaged and sold for advertising. It can anonymized and used by customer support and information technology companies. Or it can be shared with health insurers, who may use it to deny reimbursement. Privacy experts warn that data gathered by insurers could also be used to rate individuals’ health care costs and potentially raise their premiums.

Patients typically have to give consent for their data to be used — so-called “donated data.” But some patients said they weren’t aware that their information was being gathered and shared. And once the data is shared, it can be used in a number of ways. Here are a few of the most popular medical devices that can share data with insurers:

Continuous Positive Airway Pressure, or CPAP, Machines

What Are They?

One of the more popular devices for those with sleep apnea, CPAP machines are covered by insurers after a sleep study confirms the diagnosis. These units, which deliver pressurized air through masks worn by patients as they sleep, collect data and transmit it wirelessly.

What Do They Collect?

It depends on the unit, but CPAP machines can collect data on the number of hours a patient uses the device, the number of interruptions in sleep and the amount of air that leaks from the mask.

Who Gets the Info?

The data may be transmitted to the makers or suppliers of the machines. Doctors may use it to assess whether the therapy is effective. Health insurers may receive the data to track whether patients are using their CPAP machines as directed. They may refuse to reimburse the costs of the machine if the patient doesn’t use it enough. The device maker ResMed said in a statement that patients may withdraw their consent to have their data shared.

Heart Monitors

What Are They?

Heart monitors, oftentimes small, battery-powered devices worn on the body and attached to the skin with electrodes, measure and record the heart’s electrical signals, typically over a few days or weeks, to detect things like irregular heartbeats or abnormal heart rhythms. Some devices implanted under the skin can last up to five years.

What Do They Collect?

Wearable ones include Holter monitors, wired external devices that attach to the skin, and event recorders, which can track slow or fast heartbeats and fainting spells. Data can also be shared from implanted pacemakers, which keep the heart beating properly for those with arrhythmias.

Who Gets the Info?

Low resting heart rates or other abnormal heart conditions are commonly used by insurance companies to place patients in more expensive rate classes. Children undergoing genetic testing are sometimes outfitted with heart monitors before their diagnosis, increasing the odds that their data is used by insurers. This sharing is the most common complaint cited by the World Privacy Forum, a consumer rights group.

Blood Glucose Monitors

What Are They?

Millions of Americans who have diabetes are familiar with blood glucose meters, or glucometers, which take a blood sample on a strip of paper and analyze it for glucose, or sugar, levels. This allows patients and their doctors to monitor their diabetes so they don’t have complications like heart or kidney disease. Blood glucose meters are used by the more the 1.2 million Americans with Type 1 diabetes, which is usually diagnosed in children, teens and young adults.

What Do They Collect?

Blood sugar monitors measure the concentration of glucose in a patient’s blood, a key indicator of proper diabetes management.

Who Gets the Info?

Diabetes monitoring equipment is sold directly to patients, but many still rely on insurer-provided devices. To get reimbursement for blood glucose meters, health insurers will typically ask for at least a month’s worth of blood sugar data.

Lifestyle Monitors

What Are They?

Step counters, medication alerts and trackers, and in-home cameras are among the devices in the increasingly crowded lifestyle health industry.

What Do They Collect?

Many health data research apps are made up of “donated data,” which is provided by consumers and falls outside of federal guidelines that require the sharing of personal health data be disclosed and anonymized to protect the identity of the patient. This data includes everything from counters for the number of steps you take, the calories you eat and the number of flights of stairs you climb to more traditional health metrics, such as pulse and heart rates.

Who Gets the Info?

It varies by device. But the makers of the Fitbit step counter, for example, say they never sell customer personal data or share personal information unless a user requests it; it is part of a legal process; or it is provided on a “confidential basis” to a third-party customer support or IT provider. That said, Fitbit allows users who give consent to share data “with a health insurer or wellness program,” according to a statement from the company.