The unprecedented nature of Hurricane Sandy — which struck the U.S. East Coast one year ago this week — had a significant health and psychological impact on people in the region that continues today, experts say.

The storm caused more than 100 deaths, and displaced thousands of people from their homes, some of whom required medical attention for new or ongoing illnesses.

Sandy also took a toll on mental health, exposing millions of people to at least some type of short-term distress, experts say. A smaller group of people, perhaps numbering in the hundreds of thousands, were exposed to more severe conditions that could increase their risk of long-term psychological trauma, experts say.

And for some people, the psychological effects of the hurricane are just starting to surface today.

"Mental-health concerns are always underneath, [but] they can come out unexpectedly," said Christian Burgess, director of the Disaster Distress Helpline, a national hotline dedicated to year-round disaster crisis counseling.

"We've had callers recently say, 'I never realized until now exactly how much I was affected by Sandy. It's only now, a year later, that I'm starting to feel depressed,'" Burgess said.

There are also some lessons to be learned from Hurricane Sandy's impact, including potential ways to reduce fatalities from drowning — which was the main cause of death from the storm.

And while experts say emergency responders now have a much greater appreciation for the potential mental-health effects of disasters than ever before, improvements are still needed to better identify the people at risk for psychological trauma, and to deliver early, effective interventions.

Deaths and injuries

In the United States, at least 117 people in six states died as a direct or indirect result of Hurricane Sandy, according to a Centers for Disease Control and Prevention (CDC) study.

Drowning was responsible for 40 fatalities (34 percent of all deaths). Other causes of death were trauma from being crushed, cut or struck (16 percent), and carbon-monoxide poisoning (7 percent).

While drowning used to be a very common cause of death in all U.S. hurricanes, it has become a less-frequent contributor in recent years, thanks to improvements in hurricane warning and evacuation systems, said study researcher Dr. Michelle Murti, a former CDC epidemic intelligence officer. For example, the leading cause of death from Hurricane Ike in 2008 was carbon-monoxide poisoning. But drowning was the leading cause of death for Hurricane Katrina in 2005.

Drowning can occur when people in evacuation zones do not heed orders to evacuate. Of the 20 people who drowned in their homes in New York, 18 were in evacuation zones.

Murti and her colleagues said that more research is needed to better understand how effective authorities are in reaching the people who need to hear the warnings, and the reasons people do not evacuate.

"Drowning is preventable," said study researcher Rebecca Noe, a CDC epidemiologist. "The key is that emergency managers — they really need to ensure that affected persons receive, and also understand, evacuation messages," Noe told LiveScience.

A survey conducted by New York City government after the hurricane found that most people in evacuation zones (88 percent) knew they lived in an evacuation zone before the storm hit, and that 71 percent heard the announcement to evacuate. Still, 22 percent of those who reported hearing such warnings did not evacuate, compared with 32 percent of those who did not hear the warnings. The most common reason for not evacuating, the survey found, was a belief that the storm wasn't strong enough to be dangerous.

According to the CDC study, other reasons for not evacuating included a fear of looters and a lack of transportation. [End of the World? Top Doomsday Fears]

To maximize evacuations in future storms, New York City government recommends publicizing the availability of evacuation help for homebound individuals, and coordinating with advertising companies to use digital billboards to display evacuation information.

Another analysis from the CDC found that, of the people relocated to New Jersey shelters after the storm, more than 5,100 reported a health care visit — 52 percent for an acute illness; 32 percent for follow-up care, such as blood-glucose checks or medication refills; 13 percent for a worsening chronic illness; and 3 percent for injuries.

Mental-health toll

While studies have assessed the numbers of deaths and injuries from the storm, it is more difficult to estimate the storm's mental-health toll.

Although some 70 million people, across eight nations, were in the path of the storm, their experiences were very different depending on where they lived, said James Shultz, director of the Center for Disaster & Extreme Event Preparedness (DEEP Center) at the University of Miami School of Medicine.

"It wasn't a one-size-fits-all storm; it was a very, very complex set of exposures," Shultz said.

However, a Gallup-Healthways poll conducted in January this year provides some idea of the storm's mental-health impact. The poll found that among adults living in the most affected ZIP codes in New York, New Jersey and Connecticut, there was a 25 percent increase in diagnoses of depression in the six weeks following the storm. That translates to about 540,000 new diagnoses of depression.

The likelihood of developing a severe psychological condition after a disaster depends, in part, on the degree and intensity of the trauma a person experienced.

Many people who experienced a power outage — which affected at least 8.5 million people during Sandy — and transportation shutdowns endured transient stress and distress that generally resolved when the services were restored, Shultz said.

Other people, such as those who experienced a life-threatening situation during the storm, or severe losses after the storm (such as loss of their home) may be at risk for more severe conditions, including post-traumatic stress disorder (PTSD) and major depressive disorder, Shultz said.

According to the Federal Emergency Management Agency (FEMA), hundreds of thousands of homes were damaged or destroyed during Hurricane Sandy, 174,000 people received monetary housing assistance after the storm and 23,000 people sought refuge in temporary shelters.

Although mental-health needs should not be overlooked during disasters, they often are, as people are focused on getting basic needs such as food, water and power restored, Burgess said.

However, "the reality is, we never leave our emotions behind," Burgess said. "Sooner or later, it will rise to the surface," he said.

Since Sept. 11, 2001, there has been a greater appreciation for the psychological consequences of disasters, Shultz said. There has also been a greater focus on including mental-health services in disaster-response plans, Burgess said. New York City also has its own crisis hotline, LIFENET.

But more needs to be done: There should be a greater effort to promote mental-health resources after disasters, Burgess said. "People experience crisis, and are in distress in all phases of disasters," he said. "These people need to know what resources are available."

And while researchers are starting to develop early interventions that can be delivered to people soon after disasters strike, more research is needed to show that these methods are effective, Shultz said.

Follow Rachael Rettner @RachaelRettner. Follow LiveScience @livescience, Facebook & Google+. Original article on LiveScience.