Everything changed for Tanner Wendell Stewart when he was in Bulgaria in 2012.

Stewart, a Seattle-based photographer, had spotted a man holding a baby. Wanting to capture the beautiful moment, he asked the man if he could take a photo. After a brief exchange, Stewart's translator pulled him away from the man and his child.

"I said, 'What's wrong?', and [the translator] said, 'Oh, well, the man just said you can buy his baby for $50,'" Stewart said.

Stewart opens up about the life-changing interaction in the video above. He'd been in Bulgaria as a volunteer for the A21 Campaign, a nonprofit aiming to prevent human trafficking around the world, but he says that exchange truly redirected his love of photography toward changing people's lives.

"To experience human trafficking on a firsthand level -- that it's not just a statistic, that now it's actually the face of this baby," Stewart said in the video. "It's completely shifted my passion and desire to help this cause."

So Stewart raised $74,000 through the crowdfunding website IndieGoGo, in order to publish a book of his photographs -- one taken each day of the year throughout 2013 -- and with all the proceeds benefiting the A21 campaign.

In order to capture the images, many of which were taken in 20 national parks and five different countries, Stewart gave up the comfort of his downtown Seattle apartment and drove about 10,000 miles over the course of seven months. Through his Shoot the Skies book project, he's set to donate more than $50,000 to the A21 Campaign.

Above, an image of the baby Stewart spotted in Bulgaria. The baby's father offered Stewart his child for $50.

According to the A21 Campaign, an estimated 27 million men, women and children are enslaved throughout the world today, more than any other time in human history. The average age of trafficking victims is 12 years old, and the likelihood of an individual ever being rescued from enslavement is painfully low.

If you're thinking this crisis only exists in places like Nigeria, where hundreds of schoolgirls were abducted in April by extremist group Boko Haram and threatened to be sold into slavery, you're wrong.

Roughly 15,000 people are being trafficked in the U.S. for purposes of forced labor or sexual exploitation, and many states could be doing much more to prevent the atrocity.

Despite the grim facts and uphill battle to achieve justice, Stewart believes every life saved is a step in the right direction. And to take that step, you don't need to be wealthy.

"I don't think generosity is about money. I think generosity is about your heart, and about how you give things -- whether it's your time, or your art, or your money, or whatever it is," he said. "As long as you give with a generous heart and you're passionate about it, I think that's being generous."