(CNN) -- Lindsay Kujawa turned her back for just a few seconds -- but that's all the time it took for her son Ronin to fall into the churning water.
"Mom panic and fear kicked into overdrive," Kujawa wrote on her blog, Delighted Momma. "I frantically scanned the spa ... his little head was bobbing up and down trying desperately to get air. I pulled him out as fast as I possibly could."
The whole ordeal lasted about 20 seconds, Kujawa wrote. After he calmed down, Ronin seemed fine. But later that evening the little boy would be rushed in an ambulance to Children's Hospital in San Diego, his oxygen stats dropping rapidly.
Doctors told Kujawa her son was suffering from the effects of a near drowning, sometimes called secondary drowning.
Secondary drowning can happen hours after a swimmer leaves the pool.
"Often referred to by lifeguards as 'parking lot drowning,' it is rare, but happens," Alison Osinski, water safety expert and president of Aquatic Consulting Services, told CNN in an e-mail.
The Centers for Disease Control and Prevention and the Consumer Product Safety Commission don't separate active (or wet) drownings from secondary drownings for their records. About 400 pool and spa drownings occur each year in the United States that involve children younger than 15 years old, according to a recent CPSC report; more than 75% involve children under 5.
With secondary drowning, a swimmer inhales water, either due to a near-drowning incident or sudden rush of water -- as might happen when jumping from a high surface or exiting a water slide.
"It happens in seconds because it's just a matter of getting water past the vocal chords before the body's had time to react," Dr. Sarah Hoehn with the pediatric ICU at University of Kansas Hospital told CNN affiliate WAFF.
The swimmer often appears fine immediately after the event. But over time, water left in the swimmer's lungs begins to cause edema, or swelling, said Dr. Juan Fitz, a spokesman for the American College of Emergency Physicians in Lubbock, Texas. When the lungs' alveoli are filled with water, they cannot exchange oxygen to and from the blood. This causes the heart to slow as the swimmer's blood oxygen level drops.
Inhaling pool water can also cause chemical pneumonitis, or inflammation of the lungs due to harmful chemicals.
Symptoms appear 1 to 24 hours after the incident. They can include persistent coughing, shortness of breath, chest pain, lethargy, fever and unusual mood change, Osinski said.
If the swelling is caught early, Fitz said, doctors can administer oxygen and try to remove the fluid from the lungs using diuretics and positive air pressure.
If not treated, "complications will develop and will progress to pulmonary edema (evidenced by a pink frothy discharge from the victimʼs nose and mouth), hypoxia/anoxia, respiratory and cardiac arrest, and death," Osinski wrote in a report about another secondary drowning incident in 2010.
Doctors treated Ronin for oxygen deprivation and chemical pneumonitis after he arrived at Children's Hospital. He was discharged the following night.
"He is doing great," Kujawa posted Monday morning. "I have both boys in swimming lessons three times a week. ... Ronin is slowly getting more comfortable with the water and my hope is that he will learn to love the water and swimming will soon be second nature to him."
The American Red Cross recently launched a national campaign to reduce the drowning rate in 50 U.S. cities by 50% over the next three to five years. Check with your local pool for certified swimming instructors and classes.