TCS Daily

How Toxic Was My Valley

By Joel Schwartz - April 4, 2006 12:00 AM

New York and California have the most toxic air in the nation, according to a new EPA report widely covered by the news media.[1] But having the most toxic air doesn't tell us much. After all, no matter how clean the air is, somebody's air has to be the worst in the country on any given measure. What we really need to know is how bad EPA claims the air is, and whether that claim is credible.

Based on EPA's own estimates, air pollution even in the "most toxic" areas of the country poses a miniscule cancer risk. More importantly, EPA's cancer risk estimates are grossly inflated, because they depend on the false assumption that chemicals pose the same per-unit cancer risks at real-world trace exposures as they do at massive laboratory exposures.

EPA released its 1999 National-Scale Air Toxics Assessment (NATA) late last month. NATA estimates the average and range of air pollution cancer risks by county for the entire nation, based on estimates for 177 different chemicals.

According to EPA, breathing 1999 pollution levels over a lifetime gives New York State residents an average cancer risk of 68 per million people. For Californians the risk is 66 per million, and the national average is 42 per million. EPA considers any risk greater than one per million to be unacceptable, so these seem like large risks—42 times the "acceptable" level on a nationwide basis. But even at face value, these are actually small risks. For example, at 68 per million the average New Yorker faces a one-in-14,600 risk of developing cancer due to air pollution.[2]

EPA didn't include diesel soot in its estimates of national cancer risks. However, California regulators have generated such estimates for the Los Angeles area and predict that diesel soot accounts for about 70% of total air pollution cancer risk.[3] Let's make the rough assumption that the same holds true for other areas of country. If so, then the lifetime air pollution cancer risk for the average New Yorker rises to one-in-4,400.[4] Those still seem like pretty good odds to me.

What about the county with the worst cancer risk in the country? That would be Manhattan, at 454 per million if you include diesel. That's still a lifetime cancer risk of only one-in-2,200. If Manhattan has the most cancer-causing air in the country, we're in pretty good shape.

Manhattan has the highest air pollution cancer risks in the country because it is so densely populated. Packing all those people into a small area means more emissions per square mile. The average New Yorker does drive less than the average American, but not by nearly enough to make up for the higher population densities. And Manhattanites need to have just as much food and clothing trucked in as anyone else—not to mention all those Prada handbags and Bugaboo baby strollers.

About one-third of all American—330,000 per million—will develop cancer sometime during their lifetimes. On a nationwide basis, EPA's estimates therefore imply that only one of every 2,400 cancers (0.042%) is caused by air pollution.[5] While it would be wonderful if no one contracted cancer for any reason, it is clear that reducing air pollution will do virtually nothing to reduce the total burden of this terrible disease.

Of course, how you look at air pollution cancer risks depends on whether you see the glass as half full or half empty. Even at one of every 2,400 cancers, air pollution would still be responsible for more than 500 cancers per year nationwide.[6] We would all choose to avoid these cancers if we could, and in a world of infinite resources we would. But even if we could eliminate all cancer-causing air pollutants for the unlikely sum of just $10 billion per year, that would still amount to a cost of $20 million per cancer case avoided. At that price, reducing air pollution would have to rank pretty low on any list of priorities for cancer prevention.

I've shown that even according to EPA, air pollution cancer risks are already pretty low. But even these low risks are a great exaggeration of the real cancer risks from air pollution, because standard risk assessments exaggerate cancer risks from chemicals. Risk assessments are based on studies in which animals are given the so-called maximum tolerated dose (MTD). The MTD is the highest dose of a chemical that you can give to an animal without killing it.[7] Just over half of all chemicals that have ever been tested, both natural and synthetic, are carcinogenic in rodents based on the MTD, including about half the chemicals in coffee that have been tested so far.[8]

EPA's cancer risk estimates are based on extrapolation of these high-dose studies down to the comparatively miniscule exposures in the real world, on the assumption that the risk per unit of chemical is the same at low doses as at high doses. This assumption is false. High doses of chemicals cause chronic tissue inflammation. Chronic inflammation is itself a risk factor for cancer and explains why many chemicals cause cancer at very high doses, but are unlikely to be carcinogenic at low doses.[9] Furthermore, people and animals have physiologic mechanisms to defend against carcinogens, and the activity of these defenses increases with increasing exposure.[10] These defenses have to be overwhelmed by massive doses of chemicals in order to cause cancer.

Take hydrazine as an example. In Sacramento County, where I live, EPA estimates that hydrazine caused an average cancer risk of 89 per million in 1999. But based on EPA modeling, even at hydrazine hot spots the ambient hydrazine level in Sacramento County is about 0.006 parts per billion. Compare that with animal studies, where it took 1,000 parts per billion (ppb)—165,000 times the highest levels found in air—to cause an increase in cancer risk.[11] In those same studies, even 250 ppb of hydrazine—41,000 times the highest ambient levels—did not cause cancer in rodents or dogs, which makes you wonder why EPA attributes any human cancer risk at all to the trace levels in Sacramento's air.

Whatever the risk of cancer from air pollution, those risks are dropping rapidly. EPA's NATA report is based on estimated pollution levels in 1999. But ambient air pollution levels have steadily declined. If the California estimates hold around the country, diesel soot accounts for 70% of the purported cancer risk from air pollution. We don't have trend data on diesel particulate levels specifically, but total fine particulate matter dropped 15% between 1999 and 2004.[12] On-road measurements indicate that per-mile diesel soot emissions from heavy-duty trucks declined 50% between 1997 and 2004.[13]

EPA estimates that benzene is the largest contributor to non-diesel cancer risk in the U.S., accounting for 25% of the total estimated risk from air pollution (excluding diesel). But monitoring data from around the country show ambient benzene levels have been steadily declining. For example, average benzene levels decreased about 50% in California between 1999 and 2004, the most recent year of data available.[14] Other areas have also registered large declines in benzene levels.[15] Whatever the cancer risks from diesel soot or benzene, they are now far lower than they were in 1999.

A more dramatic example comes from Sacramento County. The hydrazine emissions that accounted for two-thirds of the county's total cancer risk for 1999 had already been eliminated long before EPA published its report last month.[16]

The fact that even worst-case air pollution cancer risks are tiny hasn't stopped health experts from sounding false alarms. For example, according to the Associated Press "George Thurston, a professor of environmental medicine at New York University, said the [EPA] figures are further evidence that living in a heavily polluted city such as Los Angeles or New York is roughly equal to living with a smoker. 'People living in polluted cities are at a higher lung cancer risk, and more people are noticing more cases of lung cancer in people who haven't been smokers, so the effects of environmental exposures are becoming more apparent,' Thurston said."[17]

No matter how low air pollution gets, it seems that breathing the air continues to be just like living with a smoker. If breathing contemporary air pollution is like living with a smoker, then the risks of second-hand smoke must be much milder than Dr. Thurston would have us believe.

Thurston's claim that air pollution is causing an increase in lung cancer in non-smokers also doesn't hold up. Air pollution of all kinds has been declining for decades. Thurston is therefore implicitly claiming that declining air pollution is causing increases in lung cancer risk among non-smokers. This can't be out of ignorance of air pollution trends. Thurston himself recently published a paper highlighting long-term declines in particulate levels.[18]

Thanks to EPA, we now know that air pollution poses at worst a negligible cancer risk. On a more realistic assessment of the evidence, the actual cancer risks from air pollution are at worst a tiny fraction of the already tiny risk claimed by EPA. Now if we could get journalists to pass that message along to the public, we could all breathe a little easier.

The author is Visiting Fellow, American Enterprise Institute.

[1] Among several other outlets, the New York Times, Los Angeles Times, and Associated Press covered the report's release.

[2] Calculate this as follows: 68 per million equals a risk per person of 68/106 = 0.000068. 1/0.000068 = 14,628.

[3] South Coast Air Quality Management District, Multiple Air Toxics Exposure Study in the South Coast Air Basin (Diamond Bar, CA: March 2000).

[4] In the rest of this essay I've multiplied all of EPA's cancer risk estimates by 3.33 to account for the diesel contribution. To recover EPA's original numbers, just divide my numbers by 3.33.

[5] EPA estimates a national-average lifetime cancer risk of 42 per million. Multiply by 3.33 to account for diesel to get 138 per million. 138/330,000 = 0.00042. 1/0.00042 = 2,389, or a risk of roughly one-in-2,400.

[6] At a lifetime-average cancer risk of one-in-7,200 (EPA's national-average cancer risk multiplied by 3.33 to account for diesel), air pollution would cause an average of about 530 cancers per year in a cohort of 295 million people (the population of the U.S.).

[7] In some cases there are also epidemiologic data on humans exposed to industrial doses (literally) of certain chemicals during the early-to-mid 20th Century. For example, some of the data on diesel soot comes from railroad workers exposed to high levels during the mid-1900s.

[8] B. N. Ames and L. S. Gold, "The Causes and Prevention of Cancer: Gaining Perspective," Environ Health Perspect 105 Suppl 4 (1997): 865-73.

[9] B. N. Ames and L. S. Gold, "The Causes and Prevention of Cancer: The Role of Environment," Biotherapy 11 (1998): 205-20.

[10] Ibid.

[11] Hydrazine, Integrated Risk Information System,

[12] PM2.5 data downloaded from EPA at

[13] T. Kirchstetter, D. Hooper, Z. Apte et al., "Characterization of Particle and Gas Phase Pollutant Emissions from Heavy- and Light-Duty Vehicles in a California Roadway Tunnel," 2004 American Geophysical Union Fall Meeting, San Francisco, December 13-17, 2004.

[14] Based on 11 monitoring sites around California with complete data for 1999 and 2004, retrieved from the California Air Resources Board's 2006 air pollution data CD,

[15] N. Aleksic, G. Boynton, G. Sistla et al., "Concentrations and Trends of Benzene in Ambient Air over New York State During 1990-2003," Atmospheric Environment 39 (2005): 7894-905; M. C. McCarthy, H. R. Hafner, S. M. Raffuse et al., Temporal Trends in Air Toxics (Petaluma, CA: Sonoma Technology, 2004),; R. Oomen, J. Jauser, D. Dayton et al., "Evaluating HAP Trends: A Look at Emissions, Concentrations, and Regulation Analyses for Selected Metropolitan Statistical Areas," EPA 14th Annual Emission Inventory Conference, April 12-14, 2005,

[16] Marla Cone, "State's Air Is Among Nation's Most Toxic; Only New York has a higher risk of cancer caused by airborne chemicals, the EPA says," Los Angeles Times, March 22, 2006.

[17] Devlin Barrett, "California air ranks among worst in country; State is second only to New York in EPA study based on 1999 emissions," Associated Press, March 23, 2004.

[18] R. Lall, M. Kendall, K. Ito, G. D. Thurston, "Estimation of Historical Annual PM2.5 Exposures for Health Effects Assessment," Atmospheric Environment 38 (2004): 5217-26.



The Left Tributary Media and Good News
The reporting of good news will recommence upon the swearing in of the next Democratic President of the US, assuming that the Left Tributary Media survive that long. Good news is unacceptable during Republican administrations.

Bush didn't sign Kyoto and we're all going to die!

Frank's law of 10-squared
Assume cancer risk due to trace chemicals follows LNT (Linear No Threshold) rules. (I don't believe in LNT for good reasons learned in 30 years in Radiation Therapy Research)

Assume removing 90% of a given pollutant (down to 10%,) will yield N fewer deaths at a cost of D dollars.

Getting that next 9% of pollution reduction, (down to 1%,) ain't as easy! I use a ballpark figure of 10 times as much. And, it only reduce deaths by a tenth of the first iteration.

End result? The cost per life saved has gone up 100-fold.

"I don't care how much we spend!" I hear you say.

But what actually is spent isn't dollars, but the work of scientists, engineers, etc (a significantly limited commodity,) which could be much better spent working on something with more bang for the buck.

But then, that next step in pollution reduction DOES involve a LOT of bureaucrats.

Good post, I bet the these numbers are relavent to ETS as well
ETS fear mongering and legislation continues unchecked for numbers that are even less dramatic. Typical "I want to control what you do" psedo-science combined with BS political clap-trap.

There have been tests on susceptability of animals to cancer at varying thresholds. These tests have proven that the LNT has no basis in fact.

The scientists don't want to use it, but the politicians require it.

Air pollution and cancer measurements
Further, EPA abuses and misuses statistics in other ways as they talk about air pollution. To measure the risk of industrial pollution, EPA projects the "additional deaths per million" on the basis of people living at the industrial plant gate for every minute over a normal life expectancy. They then count the number of "additional" cancer deaths per million.
What could be the probability that a person spent every hour of his/her life at the facility gate? It has nothing to do with one death per million; or one per trillion, or more. More, in fact, than the total number of people who have ever lived on earth.
Thank goodness for EPA "statistics" and "science."

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