CNG (compressed natural gas) is commonly touted as a solution to green house gases,
global warming and pollution. This has led to cities around the world purchasing hundreds of
thousands of CNG buses and other vehicles. However, an old adage warns, just because
you can’t see it, doesn’t mean it isn’t there.

Despite the rush to what’s been promoted as “clean natural gas”, the truth is somewhat more
hazy, less certain and seemingly more dangerous.  The simple fact of the matter is CNG is a
fossil fuel that must be removed from deep within the earth and then processed, refined,
transported and then finally burned.  Clean electricity CNG is certainly not.

To begin with, natural gas is primarily methane.  Methane itself is approximately 23 times
more potent than carbon dioxide as a harmful green house gas (GHG).  This means that
every kilogram of methane emitted to the atmosphere has the equivalent forcing effect on
the Earth's climate of 23 kilograms of carbon dioxide over a 100 year period. There are at
least three environmental impact factors that have not been sufficiently considered nor
researched when analyzing the total environmental impact of CNG usage.  

First, the processing and transportation of the natural gas, second, the actual emissions
from burning CNG and their toxicity and lastly, the consequences resulting from the process
of extracting the natural gas from the Earth (which not only requires millions of gallons of
waters combined with toxic chemicals, but which has been shown to result in the
contamination of ground water with carcinogenic chemicals).   


Two recent studies shed light on the actual emissions from CNG combustion in buses and
other heavy vehicles.  Both studies indicate that CNG emissions are just as toxic as gas and
diesel emissions but potentially more dangerous to living organisms.

First, in a 2005 study by the Society of Toxicity supported by the Freedom CAR and Vehicle
Technology Program of the U.S. Department of Energy, Office of Energy Efficiency and
Renewable Energy, entitled Composition, Toxicity and Mutagenicity of Particulate and
Semivolatile Emissions from Heavy-Duty Compressed Natural Gas-Powered Vehicles,
researches concluded that the emissions from CNG powered buses were in fact toxic and
contained mutagenic agents.  However, they ran into unexpected results from which they
concluded the toxicity emissions could be higher than reported and potentially more toxic and
dangerous to human lung tissue.  In fact, this result correlates with the second study in
Australia, also cited herein. In that study, the Australian scientists discovered the CNG
emissions produced just as many particles of emissions but that those particles were far
smaller than the diesel particles and therefore more likely to penetrate human tissue, which
likely explains the American researchers findings.

“In summary, these data demonstrate that collected PM + SVOC emission samples from CNG
fueled vehicles contain both mutagenic and toxic constituents, and serve to place the
potential hazard from these emissions in the context of potential health effects from diesel
and gasoline-fueled vehicles’ emissions.”

“Despite this study advancing our knowledge of the lung toxicity and mutagenicity of natural
gas emissions it has several limitations. The use of collected, acetone-extracted, and
processed samples (as opposed to a study conducted by inhalation) was a limitation of this
study. Although attempts were made to quantitatively remove 100% of the PM from the filters,
only 15–20% of the PM could be removed. This was much less than the expected efficiency,
based on results obtained using the same procedures with gasoline and diesel emissions
perhaps suggesting very different composition of the original exhaust. There may be several
explanations for the sub-optimal extraction. First, the organic solvent may not have efficiently
removed the large amounts of metallic/ionic elements that were associated with the PM from
the CNG vehicles. Second, the organic PM appeared to be stickier than previous emission
samples, making removal of the material from the filters by mechanical agitation less

If this had been human lung tissue as opposed to a filter material, the same results would
apply, e.g., the “stickier” pollutants would not only make their way into human lungs but would
be more likely to stick to the lung tissue.  

This analysis is supported by the Australian study, the results of which were published in
Science of the Total Environment.  Zoran Ristovski, from the Queensland University of
Technology in Brisbane, Australia, and colleagues compared the emissions produced by 13
different CNG buses with nine modern ultra-low-sulphur diesel buses. Each of the buses was
taken to a specially set-up test centre and run under four different conditions: idle engine
and three steady engine loads of 25%, 50% and 100% of maximum power. In all cases the
buses had a fixed speed of 60 kilometres per hour. Exhaust gases were funneled into a
stainless steel pipe and then diverted towards detectors measuring particle concentration,
carbon dioxide levels, oxides of nitrogen levels and methane levels.

The scientists were not surprised to find that carbon dioxide emissions were 20% to 30%
greater for diesel buses, while oxides of nitrogen were similar for both sorts of bus. However,
when it came to particle emissions the researchers were surprised by what they discovered.
Although the diesel buses emitted a greater mass of particles, both types of bus pumped out
a similar number of particles. This discrepancy was explained when the scientists analyzed
the particle sizes. According to Ristovski, "When it comes to nanoparticles and ultrafine
particles CNG buses emit as many particles as diesel buses.  Diesel buses tend to pump out
large sooty particles, while CNG buses emit an exceedingly fine dust.

As yet no one knows exactly what the CNG exhaust fume particles are made from, or how
toxic to human health they are. Ristovski and his colleagues discovered that the CNG
particles were more volatile than diesel particles. "This means that they are most likely
composed of some more volatile organics," said Ristovski.
One major concern is that these exceedingly fine particles (less than 50 nm diameter) may
be more toxic than larger particles because they are able to penetrate deeper into the
human lung.  This conclusion coincides with the filtration results obtained by the American
toxicologists noted above when they discovered their filtration methods were unable to
release the particles they’d absorbed and with some particles potentially flowing through the
Given the research results, it’s clear that CNG emissions are not “clean” but may well in fact
be toxic and more dangerous than gas or diesel emissions.  It’s also clear that a
comprehensive study of the matter must be undertaken given the proliferation of CNG
vehicles around the world.

According to the U.S. Department of Energy, a report generated by the Association of
German Engineers (VDI) and the results of a study by the Texas Commission on
Environmental Quality, the allegedly cleaner tailpipe emissions resulting from the use of CNG
may be neutralized by harmful emissions created in the drilling, processing and
transportation of the natural gas.  According to the German report, CNG fueled cars emit
25% less CO2 than gasoline fueled vehicles and up to 99% less particulate emissions by
mass.  However, according to Professor Hans Lenz, the CNG in total is no better in the final
analysis because there are significant emissions created in the processing and transporting
the fuel. The pollution comes from several major sources including the tank batteries used to
store wastewater and condensate (which is a light form of crude oil which often vents to the
atmosphere); the motors used to drive pipeline compressors often have no pollution controls;
drillers frequently vent natural gas to the atmosphere when they complete a well; and the
process of liquefying the gas for transport in LNG tankers.
The Department of Energy concedes that, taking into account the processing emissions of
natural gas, when comparing total diesel emissions and natural gas emissions the end
results are about the same.  (Department of Energy, Natural Gas Buses: Separating Myth
from Fact.)

Similarly, a Texas environmental studies report, released in June of 2009, showed that
natural gas drilling contributed as much air pollution to the Dallas-Fort Worth area as car and
truck traffic combined.  Al Armendariz, an engineering professor at Southern Methodist
University and author of the original study, estimated that, in the nine-county Metroplex area,
gas drilling produced about 112 tons per day of pollution, compared with 120 tons per day
from vehicle traffic. In a 20-county area, including rural counties, he estimated that gas
drilling produced 191 tons per day.  His studies results were largely confirmed by the Texas
Commission on Environmental Quality in their follow-up study.

From the evidence presented it would appear as though CNG powered vehicles do not
reduce the overall impact on global warming.  They merely shift the point of emissions.  

Natural-Gas Drilling Endangers Water Supplies

90% of the natural gas obtained is via a process known as hydraulic fracturing or
hydrofracking.  Hydrofracking involves shooting millions of gallons of water and drilling
chemicals at explosive pressures deep underground to break up the rock.  It bears noting
that, aside from the ground water issue, vast quantities of water used in the aforesaid drilling
process are contaminated with carcinogens.  

In 2005, under the Bush-Cheney Administration, Congress exempted hydraulic fracturing
from the Safe Drinking Water Act. That effectively eliminated EPA jurisdiction over the drilling
technique and left oversight to state regulators and, in the case of federally owned land, the
U.S. Bureau of Land Management (BLM), an agency often characterized as friendly to
industry.  This lack of oversight has led to what appears to be a hazardous situation.

Congress relied on a 2004 EPA study, but that 424-page report's conclusions appear, on
close examination, to ignore some of its own findings. The report actually notes that
fracturing fluids migrated unpredictably through rock layers in half the cases studied in the U.
S.  The agency characterized some of the chemicals as biocides and lubricants that "can
cause kidney, liver, heart, blood, and brain damage through prolonged or repeated
exposure." The report also noted that as much as a third of injected fluids used in hydraulic
fracturing remains in the ground and are "likely to be transported by groundwater."

Before becoming Vice President, Dick Cheney was the Chairmen and CEO of Haliburton.  
Haliburton, one of the primary natural gas extractors, refuses to disclose the cocktail of
dangerous chemicals it shoots deep into the earth.   

The potential impact of this practice is widespread.  In 2007 there were 449,000 gas wells in
32 states. As gas drilling has spread—and encroached on residential areas in some states—
new questions are surfacing about whether the fluids used in fracturing cause water
contamination. According to the Interstate Oil & Gas Compact Commission, a multistate
government agency, nine out of 10 gas wells in the U.S. rely on the fracturing technique. The
concern is that hazardous chemicals may escape from some well sites as a result of leaky
waste pits, spills caused by worker negligence or underground leaching.

Serious episodes of water contamination near drilling sites have been documented in seven
states: Alabama, Colorado, Montana, New Mexico, Ohio, Texas, and Wyoming. Numerous
incidents of contamination have occurred in western Colorado where drilling has expanded

In 2004 a well casing shattered beneath a rig at Divide Creek, a tributary of the Colorado
River, which supplies water to seven states. Dangerous levels of benzene turned up in
groundwater and stream samples, state records show. Benzene is a carcinogen, according
to the EPA, and has been linked to aplastic anemia and leukemia.

In June a rancher in Parachute, Colorado, was hospitalized after he drank well water from his
tap. Tests showed benzene in his water. The Colorado Oil & Gas Conservation Commission
blamed four gas operators in the area for spilling waste fluids. An investigation is continuing.

Pointing to such episodes, several experts in the EPA's regional office in Denver have begun
to raise questions about the agency's conclusion in its 2004 report that hydraulic fracturing is
safe. "We've kind of reached the tipping point where the impacts are there," says Joyel
Dhieux, an EPA scientist in Denver who reviews the effects of industrial projects.

However, one of the report's three main authors, Jeffrey Jollie, an EPA staff hydrogeologist,
cautions that the study was narrowly focused and has been misconstrued by the gas-drilling
industry. The study looked at the effects of fracturing in so-called coalbed methane deposits;
it did not consider the above-ground impact of drilling or what goes on in many of the large
new gas reserves being developed today.

"It was never intended to be a broad, sweeping study," Jollie says. "I don't think we ever
characterized it that way."

In rural Sublette County, Wyoming, an area the size of Connecticut with two mountain ranges
but no stoplights, recent testing by federal and state officials near one of the nation's largest
gas fields found 88 contaminated water wells stretching over 28 miles.  Fifteen contained
benzene, in one case at more than 1,500 times the amount the EPA says is safe, according
to the Bureau of Land Management.

The EPA's Denver-based regional water expert, Gregory Oberley, states: "You've got
benzene in a usable aquifer, and nobody is able to verbalize well, using factual information,
how the benzene got there."

Of the more than 300 chemicals thought to be in use by drillers, more than 60 are listed as
hazardous by the federal government.

Drilling for gas is now planned for the region of New York State that supplies New York with
its drink water.  New York is one of just four major cities in the United States with a special
permit allowing its drinking water to go unfiltered, and that pristine water comes from a
network of reservoirs and rivers in five upstate counties. If the special permit was revoked,
the city would have to build a treatment facility that could cost nearly $10 billion, states
Walter Mugden, a senior official at the U.S. Environmental Protection Agency

The City of New York views this drilling as a direct threat to its critical water resources and is
taking measures to oppose the planned drilling.

As one commentator noted, we can survive without the natural gas, we can’t survive without
drinkable water.

Until such time as drilling practices are changed to remove the threats to our ground water,
these present dangers must be considered as an elemental environmental cost of
consuming natural gas.