Envelopes filled with white powder are sending people scampering to the emergency room. Hot air balloon races are prompting calls to the police. Even fireworks shows are sparking fears.

America has a bad case of the jitters.

"We’re on edge," says social psychologist Kenneth D. Richardson. "It’s perfectly understandable. We’re all on high alert right now."

A month ago it was flying. Last weekend, it was terrorist retaliation for the military strikes in Afghanistan. Now, it's hysteria about anthrax and the threat of bioterror following news that a Florida man died from exposure to the bacterium and a second person who worked in the same office building tested positive for it.

"If you combine the Sept. 11 attacks with the anthrax scares, the salience of these events is being heightened to create excessive fear," said psychology expert Michael Wessells, a professor at Randolph-Macon College in Virginia. "But if we allow ourselves to be controlled excessively by fear, haven't we allowed the terrorists to win?"

Suddenly, what seemed unimaginable five weeks ago is not so fantastic in the eyes of many Americans — and they are reacting, or overreacting, accordingly.

In a Washington, D.C., suburb, a man sprayed a substance into the air during a scuffle with police, then dropped a jar of clear liquid on the platform. Police shut down the station. Several people reported feeling sick, but a fire department spokesman said the substance sprayed was cleaning solution.

Anthrax scares have become almost common across Florida, including in Naples and Miami, where a handful of people received envelopes containing a white powder.

Two families in Miami found envelopes filled with an unknown substance in their mailboxes, and in Naples the envelopes arrived at a bank and a law firm. Several people went to the hospital because they said felt ill and police were testing the substances, though they guessed the powders in question weren’t toxic.

In Cincinnati, authorities went so far as to quarantine employees and customers at a highway restaurant called Frisch’s after a small vial containing a clear liquid was left in the restaurant overnight.

Psychologists said some of the jitters are natural considering the unfamiliar ground most Americans find themselves on lately.

"It’s alien turf we’re on," Richardson said. "And the uncertainty is just adding to it. It hangs over you. You wish it would go away — but it won’t."

But they warn against unnecessary or exaggerated fear that has little basis in reality.

"The concern I have is that a sort of hysteria will set in where people will think that terrorism is imminent in nearly every place at every moment of every day," Wessells said. "That does not fit the evidence."

The national panic that has taken root was underscored by reactions to occurrences that would have gone unnoticed a month ago.

A fireworks show in Medford, Mass., to celebrate the opening of two new public schools sent skittish residents pouring into the streets and jamming the 911 lines there over the weekend.

"It actually did sound and look like bombs," said Virginia Ferriman, whose house is a block from where the fireworks were launched. "My husband jumped off the couch. We were running around, bumping into each other. At the time it wasn’t funny. We were in a panic."

And in the Midwest, people in Iowa and Minnesota called the police when they saw what they reported as suspicious-looking hot air balloons flying overhead. In fact, the gas-filled balloons were part of the America’s Challenge race that began in Albuquerque, N.M., a few days earlier.

Experts advise Americans to try to calm down, stay grounded, comfort each other and get the facts straight instead of panicking over what could turn out to be a rumor or a false alarm.

"It's time for reality testing, solidarity and support," said Wessells. "There's no reason why one should expect a terrorist behind every tree and under every rug at this point."

The Associated Press contributed to this report.