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Anyone who stood waiting for a bus 20 years ago may remember noxious fumes spewing from the tailpipe.
Those days are disappearing as more buses are being fueled by compressed natural gas than by gasoline and diesel, said Kevin Kilpatrick, assistant general manager at River Valley Transit.
"We have 19 buses running on compressed natural gas, or a little over half of the fleet," he said.
That's one breath of fresh air worth noting today -- Earth Day 2018.
The first Earth Day occurred under President Richard M. Nixon's administration, as people became aware they were temporary stewards of the planet.
Today, compressed natural gas is quickly becoming the go-to product instead of petroleum for the bus service, according to Kilpatrick.
The gas produces no detectable toxic fumes, is safer in the event of a spill, is lighter than air, disperses quickly and may be stored in hard containers, he said.
Consisting mostly of methane, the gas is odorless, colorless and tasteless.
"Methane stored at high pressure," said Matthew Nealis, project manager with Larson Design Group and a professional engineer.
It's greener and cleaner burning because it consists chemically of one part carbon and four parts hydrogen, Nealis said.
When hydrogen is burned, it becomes water vapor, he said.
Conversely, diesel and petrol-based fuels contain far more carbons. When those are burned, the emissions contain Greenhouse gases that have been shown to damage the Earth's fragile ozone layer, he said.
By comparison, a compressed natural gas vehicle engine has between 15 to 20 percent fewer emissions.
New technology is starting to be distributed, he said.
A near-zero-emission engine was created by the firm Cummins Westport.
The engine meets emissions certification for release of nitrous oxide limits established by the U.S. Environmental Protection Agency and Air Resources Board in California, Nealis said.
Exhaust emissions on the engine are 90-percent lower than federal government maximum limits, he said.
Another type of pollution is reduced by using compressed natural gas -- noise.
The noise levels for compressed natural gas trucks are 90 percent lower compared with diesel-powered vehicles, Nealis said.
Noise from one idling diesel truck is equivalent to 10 idling compressed natural gas trucks, he said.
The vehicles run on gas extracted from shale below the Earth's surface but the new push is for renewable gas pulled off landfills or cattle farms, Nealis said.
"If you think about it, manure and landfills have methane released into the atmosphere," he said. "Collecting it and using it for transportation is the most green or environmentally friendly fuel source right now."
Another source of the gas is from wastewater treatment plants, he said.
"Hopefully, the state lawmakers and administration can push for federal government credits to incentivize companies to seek renewable natural gas," Nealis said.