Scientific American 201905

6 Scientific American, May 2019

LETTERS

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CARBON CALCULATION
In “The Last Resort,” Richard Conniff
states that “our cars each typically emit
4.6 [metric tons, or about five U.S. short
tons] of carbon dioxide a year.” I would
ap pre ciate an explanation as to how this
figure was derived. Assuming an average
vehi cle is 4,000 pounds, I find it hard to
be lieve that in a given year, an automobile
produces more than twice its weight in
carbon dioxide.
For example, if a car drives an average
of 10,000 miles a year and gets 20 miles
per gallon, it will consume 500 gallons
of gasoline in a year. Gasoline weighs
around six pounds per gallon, so if the
assertion from the article is correct, that
means burning 3,000 pounds of gasoline
would produce more than three times its
weight in  CO 2 . 
John Cardwell
Supply, N.C.

CONNIFF REPLIES: Cardwell’s question

brings up one of the facts about green­

house gas emissions that I found hardest

to fa thom when I first encountered it: al­

though a gallon of gasoline is indeed six

pounds, we put more than 19 pounds of

CO 2 into the atmosphere for every gallon

that our cars burn (not counting addition­

al emis sions released by manufacturing

and trans porting the fuel to market). How

could that be?

Gasoline is nearly 90 percent carbon.

And through combustion, nearly every

carbon atom in the gas combines with two

oxygen atoms. Oxygen is 1.33 times heavi­

er than carbon. Thus, for six pounds of

gas, more than five pounds of carbon com­

bines with around 14 pounds of oxy gen.

The Environmental Protection Agency ar­

rived at the figure for a vehicle’s an nual

emissions by assuming the average car

has a fuel economy of about 22 miles per

gallon and drives about 11,500 miles each

year. That adds up to about 523 gallons

and produces a dismaying annual total of

more than 10,000 pounds, or about 4.

metric tons, of CO 2.

FIT INHERITANCE

“Evolved to Exercise,” by Herman Pontzer,

discusses research showing that over two

million years human physiology adapted

for a high level of physical activity.

I wondered if there is a relation be­

tween our past of body exertion and our

hair lessness relative to great apes. Hu­

mans can sweat copiously to maintain

coolness, which must be done during

bouts of prolonged exercise. I would think

that sweating would not be nearly as effec­

tive soaking through a thick coat of fur. My

take is that as we evolved to undergo in­

creasing exertion under a hot sun, we con­

comitantly lost more and more body hair.

Barry Siler

Loveland, Colo.

Pontzer states that “our taking fewer

than 10,000 daily steps is associated with

increased risk of cardiovascular and me­

tabolic disease.” That so­called bench­

mark value of 10,000 steps a day gained

popularity in the media mostly because

it simplifies matters, and it can be traced

back to a marketing slogan for a Japa­

nese pedometer decades ago. Three re­

cent studies, in which I was not in volved,

reviewed existing evidence for bench­

marks in human physical activity, finding

large differences between needs for dai ­

ly activity among children, ado lescents,

adults and older people, and those with

chronic diseases.

I understand the need to be concise

in a magazine article. But reproducing a

one­size­fits­all benchmark number for

all groups in the population, which is not

based on scientific research, could possi­

bly be damaging to public health.

Arno Maetens

Doctoral researcher in social health

sciences, Vrije University Brussels

PONTZER REPLIES: Regarding Siler’s

suggestion: Skin doesn’t fossilize, but most

paleontologists would agree that hairless­

ness and sweating likely evolved along

with increased physical activity early in

the genus Homo. Humans are the sweati­

est animals on the planet, and our ability

to stay cool allows us to keep going in con­

ditions that make other mammals melt.

The 10,000 steps per day benchmark

widely used in public health is a nice,

round number that’s easily remembered,

but Maetens is quite right that it’s not nec­

essarily the best fit for all populations.

Children should be getting more than that

(11,000 to 15,000 is a good goal), while old­

er people and those with limited mobility

can aim for less. The good news for any­

one anxious about the perfect number of

steps for them is that more is almost al­

ways better. Barring any health condi­

tions that are aggravated by exer cise,

there’s no evidence that too much walking

is ever bad for you, re gardless of what my

kids say when we’re out hiking.

UNOBSTRUCTED PAIN

“What Ails a Woman’s Heart,” by Claudia

Wallis [The Science of Health], discusses

research by cardiologist C.  Noel Bairey

Merz and others on INOCA (ischemia and

no obstructive coronary artery dis ease),

a condition in which patients with out

blocked arteries nonetheless experi ence

poor blood flow through the heart.

If ischemic chest pain can be caused

January 2019

“ R e p r o d u c i n g a

one-size-fits-all

benchmark number

for daily steps

for all groups

in the population

could be damaging

to public health.”

—arno maetens vrije university brussels

© 2019 Scientific American