Trevor Hughes

USA TODAY

DENVER — There's a little something extra drifting through the Colorado air these days: Marijuana.

The lush, skunky smell of growing pot hangs heavy over certain sections of the Mile High City as dozens of legal indoor grow operations turn electricity, water and fertilizer into mature marijuana plants. Those grow houses inside unmarked warehouses fitted with security cameras, heavy-duty electrical connections and shiny new ventilation systems are helping meet the demand for legal marijuana.

Long confined to isolated areas far from prying eyes and sniffing noses, the marijuana industry has gone mainstream, and that you-can-smell-it presence has upset some people. This year, about 30% of the smell complaints coming into Denver's code enforcement office are about the pot smell coming from the largely industrial areas away from most homes, schools and parks.

"No one ever complains about bakeries," said Ben Siller, a Denver code enforcement officer with the Department of Environmental Health. "I'd laugh if someone complained about cinnamon rolls, but it doesn't happen."

Siller has received international attention for his use of what's known as a "Nasal Ranger," a special device that allows him to scientifically measure how strong an odor is. The device, which Siller allowed a USA TODAY reporter to test, combines specially filtered air with the outside air in measured increments. Even though the city has received dozens of complaints about pot smells, Siller has not yet measured any marijuana smell that violates the city's standard — no more than 1 part odor to 7 parts filtered air or five complaints from nearby residents within a 12-hour period.

He chalks the complaints up to people still getting used to the idea that marijuana is legal.

"You do have people who just object to the whole idea," he said. "(The smell) is discernible. It's there but you get used to it, just like any odor."

Marijuana growers say they understand neighbors' concerns and take steps to filter their grow-room exhaust. When marijuana was illegal in Colorado, growers often took multiple steps to filter the air coming out of their grow rooms in an effort to hide their existence.

Indoor growers often pump the room full of carbon dioxide, which helps the plants grow bigger and faster, but that requires massive quantities of air to be pumped through the facility. The filters on those illegal grows were designed to totally remove any marijuana smell — to keep from tipping off unsuspecting neighbors or patrolling cops.

Legal grow operations are allowed to emit a certain amount of odor, just like any other manufacturing operation or farm that fertilizes with manure. At Denver's Discreet Dispensary, workers have installed special carbon filters to help head off neighborhood complaints, even though the store and grow operation sits amid an industrial area.

"It's just a flower," said Kurt Britz, the company's head of security. "They're afraid of a smell — but right up the street we have a dog food factory."

The conflict isn't just confined to industrial areas. In nearby Boulder, some rural residents are upset about a marijuana grow operation that is proposed for a farming area a few miles outside the city.

Farmer Bob Munson said many schoolchildren visit his property to learn where their food comes from, and local chefs host farm dinners there in the evenings. He said having a grow operation start up would change the character of the area.

"If there was a marijuana smell ... that would be pretty hard to explain to the kids and the families that came that there was a marijuana grow operation there," said Munson, who got his start growing organic vegetables fertilized with chicken manure. "It isn't appealing in an area like this."

Siller said the number of complaints about marijuana smells in Denver appears to be slowing down as people get accustomed to the new scent in the air. He said he encourages grow operations to voluntarily install better odor controls whenever possible, to help reduce the concern.

"For the most part, they want to be good neighbors," he said.