Every Fourth of July, Tom McKenna has to prepare himself.

McKenna, 48, is among U.S. military veterans who have been diagnosed with post-traumatic stress disorder. For him, fireworks sound like the explosions of combat — loud, unexpected and heard in the dark.

"They can be very uncomfortable and can cause flashbacks that can cause agitation and throw me off," said McKenna, a Marine Corps veteran who served in Somalia and Iraq and now works with the veterans support organization Every Third Saturday.

Seven of 100 veterans will have PTSD at some point in their life, according to the U.S. Department of Veterans Affairs. Rates are higher for those who served in recent conflicts, including the Persian Gulf War and the wars in Afghanistan and Iraq.

Fireworks also can trigger other groups of people with PTSD, including victims of gun violence and refugees who've experienced violent conflict. The St. Paul-based Center for Victims of Torture recommends that people be aware of their refugee neighbors, give them advance notice of fireworks or parades and avoid setting off fireworks in their neighborhood.

McKenna reminds himself every year that it's likely he'll hear fireworks leading up to and on July 4th. To drown out the noise, he'll turn up the TV or listen to an audiobook. His wife will sit by his side on the couch, ready to reassure him with the words, "It's just fireworks."

"It kind of starts the week prior to the Fourth of July and might go a few days after and can be very jarring, especially the big ones, most of which should be illegal in Minnesota," McKenna said.

Consumer-grade fireworks such as firecrackers, bottle rockets and Roman candles are illegal in Minnesota, but that doesn't stop people from traveling to other states, including Wisconsin, to buy them.

Sgt. Mike Ernster, a St. Paul Police Department spokesman, wrote that the department has seen an increase in illegal fireworks in the past five years. Possessing illegal fireworks is a misdemeanor punishable by up to 90 days in jail or a fine of up to $1,000, or both.

St. Paul police frequently get complaints about fireworks, Ernster said, and ask residents to be aware of how setting off fireworks can affect their neighbors.

"Our veteran population who have served in our armed forces are one of those groups who are deeply affected," Ernster said. "The sounds of the explosion can cause emotional and psychological reactions."

Officers will be on patrol in the St. Paul area on July 4th to keep an eye out for people using illegal fireworks, he said.

McKenna said he wants people to celebrate Independence Day, but also wants them to be cognizant about how they do it.

"I just wish people would understand how unsettling [fireworks] can be," he said. "People want to have fun and enjoy the holiday. At the same time, it's not necessary lighting off the things that really have this big boom."