He says it is "by far the worst" of the child labor conditions he has seen. "It is truly one of the worst hazardous experiences of the environment, which children can be faced with -- not just from the short-term hazard that they face but the long-term health consequences."
Gutierrez makes about $2.50 at the end of each day. Like thousands of families, his family is trapped in a cycle of poverty. The child miners of Potosí start young and the die young. Most don't live to see 40.
Gutierrez's father is still alive. Like almost all of the veteran miners his lungs are in poor shape after years of breathing the dusty mine air without safety masks. He can't work in the mines, so his three sons must. It has been like this for generations.
"Children in many cases supply the income that allows people to meet their basic needs just to survive," Trew said.
Gutierrez's mother, Doña Alejandra Caleapa de Gutierrez, struggles to keep food on the table. Each morning while her boys head to the mine, she prepares lunches she hopes to sell to other miners at a tunnel entrance to make a few extra dollars.
As Gutierrez leaves the modest cinder block house on dirt hillside overlooking the colonial city, he pauses at the gate to bless himself. He lives in fear of the day ahead.
His mother told us she worries every day when her son leaves for the mines. "It makes me so sad to see him working there," she said in Spanish, tears running down her cheeks. "Anything could happen, it is so dangerous."
The day we met Doña Alejandra she learned that her oldest son had been injured when a rock slab fell on him in the mine.
"I don't want my boys to work there. I worry more than anything about them. I want them to have a steady job, to be secure."
But in the highest city on Earth, in the poorest province of the poorest country in South America, there is only one option for most families: the mine.
Early each morning the miner's market buzzes with activity as the men and boys search for essential supplies: food, head lamps and dynamite that sells for 25 cents a stick. There are crude safety masks for sale for about $12 but few wear them. They're expensive, but a lack of safety awareness also seems to allow miners to think that things like breathing masks are unmanly.
Gutierrez says sometimes when he's in the mine he can feel the deadly dust going inside his body.
"I feel dizzy," he said, "as if I am going to faint. Sometimes we tie a handkerchief around our mouth so we don't breath it in."
Gutierrez and his brother are used to thin air at this altitude, but the miners never get used to life inside the mine, at least not without help from the coca leaf. Coca is the fuel that has kept this mine running for all these centuries. It comes from the lowland valleys of Bolivia. The leaf has to go through a complex chemical process to become cocaine, chewed raw it is simply a stimulant that suppresses hunger, fear and fatigue.
Gutierrez says he couldn't do the job without coca. "If you don't use coca, you can become anxious inside. You can become weak. Coca keeps you up even if you work all night."
Inside the mine they're not looking for silver anymore; they'll settle for scraps of tin and other minerals. The tools they use have hardly changed since the Spanish conquerors left: hammers and picks. There are drills much deeper in the mine, but Gutierrez doesn't use them.