(DANIEL OCHOA DE OLZA/ASOCIATED PRESS)

They call swimming a low-impact sport because it’s easy on the joints. But sometimes it’s not so easy on the brain.

We’ve heard a lot in recent years about the dangers of head trauma in sports, especially football, soccer and ice hockey.

But swimming?

You wouldn’t think so, but just ask avid swimmers Kate and Maria (who asked that their last names or other identifying details not be used to protect their privacy), both of whom suffered concussions recently at a Washington area pool, each apparently the result of inappropriate behavior by other swimmers. Maria’s injury was so serious that she missed nearly three weeks of work. “I’ve had headaches and I am sensitive to sound,” she says, and, even after several months, “I’m still not back to my usual routine.”

People who swim regularly know the protocol when doing laps. If you enter a lane that already has a swimmer in it, for example, you’re expected to alert that person that you are coming in. Typically, when two share a lane, each swimmer takes a single side and sticks to it. With three or more swimmers, you’re supposed to “circle swim,” which means that everyone stays to the right. If you must pass, you should do it carefully and not try to overtake someone who is approaching an end of the lane.

That’s how Kate got hurt. She was one of three swimmers circling. When she reached the wall, she turned “and suddenly felt a crashing blow to my head,” she says, colliding with the swimmer behind her. “I held my head and realized that the hit was hard,” she recalls. “I’m not sure which body part hit me. . . . Startled and a bit dazed, I stood up and asked, ‘What was that?’ ”

The other swimmer “paused for a brief moment, accused me of not letting her pass, claiming that she had ‘touched my foot’ (a signal that alerts a swimmer of someone who is about to pass), although I felt nothing of the kind,’’ Kate says. “She did not ask if I was okay.’’

Fortunately, her concussion was mild and, after speaking with her doctor, she spent the day resting and subsequently recovered.

Maria wasn’t as fortunate. Alone in her lane, she was keeping to one side of the lane when another swimmer entered and began swimming on the other side, which was appropriate. But instead of staying on that side, she circled into Maria’s side and swam right into her, their heads hitting. Maria was swimming backstroke and didn’t see her. The swimmer who hit Maria apologized, “saying she was new and did not know’’ she was supposed to stay on the same side, Maria says.

Maria’s family insisted she see a doctor, who, based on the persistent headaches that followed the accident, diagnosed a concussion. Maria required weeks of extended rest and a break from exercise, computers, television and reading. She still hasn’t recovered from the August incident. “The treatment is rest for as long as it takes my brain to heal,” she says.

To be sure, concussions in swimming are rare compared to those in such sports as football, soccer and hockey. But it’s easy to see how they can occur when a swimmer ignores etiquette. Swimmers’ faces are in the water. Goggles get foggy. And, at outdoor pools, the sun can be blinding.

The Centers for Disease Control and Prevention collects data on sports concussions, but most of it comes from high schools and the NCAA. “We just don’t know what’s going on outside organized school sports, including among adults, although we believe the risk in swimming and diving to be fairly low,” says behavioral scientist Matt Breiding, who leads CDC’s traumatic brain injury team, part of its division of unintentional injury prevention. “We are trying to put together a national concussion surveillance system to get a better estimate.”

The CDC is planning a pilot study to start next year that will survey an estimated 10,000 households about sports-related concussions. “Hopefully we will then have a better picture of sports-related concussions outside high schools and colleges, including among adults,” Breiding says.

Kate and Maria believe that their accidents could have been prevented by more-vigilant lifeguards. They think lifeguards should watch for potentially dangerous interactions — such as two people swimming toward one another in the same lane — and intervene to avoid a collision.

“They should be trained to proactively identify potentially harmful situations,” Maria says.

Kate agrees. “They need to be aware of the dynamics between swimmers at all times,” she says.

B.J. Fisher, director of health and safety for the American Lifeguard Association, says pools must establish and post rules for lane behavior. The pool where Kate and Maria swim posts rules stating that more than two swimmers must circle, with an illustrated diagram, but makes no mention of appropriate protocol when only two swimmers are in a lane and instructs swimmers to pass with care in the center of a lane.

“The facility needs to establish good safety rules that patrons can follow and lifeguards can enforce, and the rules should be written and posted,” he says. “Also, if there is an injury, the lifeguard needs to step in and consider it serious. If someone has had a head collision, don’t let that person just walk away, even if he or she says he or she is fine. Call 911 if you have to, and get medical help.”

Lifeguards did not do so in the cases of Kate and Maria. “I wasn’t able to continue swimming, and hobbled out of the pool,” Kate says. While guards were on duty at the time, Kate says, she didn’t see one nearby, and they did not respond to the incident.

Maria’s experience was similar. “I believe my concussion could have been prevented had the lifeguard been paying attention,” she says. When she complained to the guard, “his response was ‘I don’t know what I could have done; swimmers are supposed to look out for themselves,’ ’’ Maria says, adding: “I’m not sure how I was supposed to do this. How could I have known this person was behind me?’’ Moreover, “he did not ask me if I was okay or if I needed medical assistance.’’

The accidents have altered how they feel about swimming. Kate is still swimming daily workouts, but is nervous. “I am very cautious, and have become acutely aware of the presence of others in my lane,” she says. Also, “I avoid swimming with fast, aggressive swimmers.”

Maria has stopped swimming entirely. “Swimming was my favorite hobby, but I have been traumatized by the incident and have not returned to the pool,” she says. “I’m not sure when and if I ever will.”