Across the nation, hospitals and doctors' offices are returning blood pressure cuffs to their manufacturers to comply with a federal environmental initiative to cut down on the use of mercury, a toxic metal that can pollute the air and water when disposed of improperly.

But leading medical experts, joined by the American Heart Association and the National Heart, Lung, and Blood Institute, say the mercury gauges are being replaced by newer devices that may be unreliable, and they warn that inaccuracies may be leading to false diagnoses and inappropriate treatments.

Nobody knows how many hospitals and doctors have abandoned the old blood pressure cuffs. Nor is it known how well the newer devices are performing. Anecdotal evidence from doctors suggests that erroneous pressure readings have caused illness and death, but some medical centers, including the Mayo Clinic, say other devices have been reliable.

Still, experts are alarmed enough to call for a pause in the rush to install the newer devices. In a joint statement, the heart association and the Heart, Lung, and Blood Institute (one of the National Institutes of Health) say that before the old mercury devices are removed, the nation needs a system to be sure the new ones are accurate and reliable.

"We've been caught off guard," said the statement's author, Dr. Daniel W. Jones, a blood pressure expert at the University of Mississippi Medical Center who is a spokesman on high blood pressure for the heart association. "We don't have a system in place to ensure the accurate determination of blood pressure."

Dr. Claude Lenfant, director of the Heart, Lung, and Blood Institute, agreed, saying, "It is clearly a matter of public health."

The push to get rid of mercury sphygmomanometers, as the devices are known, began in June 1998, when the Environmental Protection Agency and the American Hospital Association agreed to eliminate as much mercury waste as possible from hospitals by 2005. Others, including medical clinics and many doctors' offices, joined the effort.

There are several alternatives to the mercury gauges, all producing the familiar numerical readings of pressure when the heart beats and when it rests between beats. Aneroid devices - the term means "without liquid" - use metal that acts like a spring to measure blood pressure. Automatic devices measure pressure in a variety of ways, including converting pressure readings into measurable electronic waves.

Edward Wright, a vice president at Welch Allyn Inc., which makes all three types of blood pressure devices - mercury, aneroid and electronic - says all are effective. "These are highly accurate instruments when they are shipped," Wright said. "In our experience, they all work very well."

But the mercury device has long been the gold standard. It measures force with a column of mercury. The mercury is pushed down by gravity and up by the blood pressure, and since gravity does not change, the gauge does not wear out. If properly maintained, it can give accurate readings for decades.

Jones, speaking for the heart association, says it is still the preferred device. But now, he added, "for the first time in 100 years we are faced with some places not having mercury manometers available for blood pressure measurements."

Dr. Edward Roccella, coordinator of the institute's National High Blood Pressure Education Program, says it is not required that the devices be proved accurate under constant use. Nor must they be proved reliable for patients of a variety of ages, illnesses and sizes. Without vigilant monitoring, there is no way to know when they go awry.

Metal in aneroid devices can fatigue, making readings inaccurate. So manufacturers recommend that they be recalibrated every six months or so, often asking that they be returned to the companies for service. But Roccella says that is seldom done. One study, he said, indicated that 30 percent of aneroid devices in use in doctors' offices and hospitals were inaccurate.

In the meantime, hospitals and medical centers are switching from mercury devices and are making their own decisions about how carefully to assess their equipment.

Some say that they have been able to rid themselves of mercury sphygmomanometers with no problems.

The Mayo Clinic switched to aneroid devices and checks them annually, said Dr. Sheldon Sheps, an emeritus professor of medicine there. He said a study by the clinic's doctors looked at 248 aneroid devices, 17 percent of the 1,500 in use at two Mayo Clinic hospitals. Only one needed to be replaced.

Others were less sanguine.

"Sometimes these devices give readings that are very wrong," Jones said, adding that the errors can be of 30, 40, even 50 points.