Dear Cecil: A coworker who considers herself "green" rides a scooter to work, confident she's leaving a smaller carbon footprint than us slugs who drive cars. But her ride has a two-stroke engine. I recall hearing that running a lawn mower for 30 minutes pollutes more than commuting all week in a car. What's the deal, Cecil? Which pollutes more per mile, a 15-MPG SUV or a 75-MPG oil-burning Vespa? Richard Chuttke, Westmont, Illinois

Cecil replies:

Two separate questions here, bud: (1) Which vehicle pollutes more? (2) Which leaves a smaller carbon footprint? You get a different answer depending on which one you ask.

The serious vehicle pollutants these days are carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons. (Sulfur and lead emissions, once problematic, are now largely a thing of the past.) Here are the federal standards for four wheels versus two:

Cars and light-duty trucks (including SUVs). Maximum CO emissions: 7.5 pounds per thousand miles. Unburned hydrocarbons: 0.154 pounds. NOx: 0.154 pounds.

Scooters and small motorcycles. Maximum CO: 42.57 pounds. Unburned hydrocarbons: 3.55 pounds. NOx: no limit.

In other words, scooters can legally emit about 5.7 times more CO than cars, nearly 24 times more unburned hydrocarbons, and infinitely more NOx and real-world testing suggests they do run pretty dirty. One reason for the looser restrictions is that it’s tougher and costlier to put emissions controls on a scooter than on a car. So while your coworker may think she’s doing the world a favor, when you look strictly at what’s coming out the tailpipe, she’s a pollution-spewing pig.

But maybe that’s OK. Let’s look at carbon footprint. The major greenhouse gas produced by motor vehicles is carbon dioxide, which is a function of gas mileage. SUV fuel economy for model years 2008 and 2009 ranges from 12 to 32 miles per gallon, for an average of 18.5. For scooters the range runs from 33 to 140 MPG; we’ll say the average is 75. At 19.4 pounds of CO2 per gallon of gas, an average SUV emits 1,050 pounds of CO2 per thousand miles, while the average scooter emits 259 pounds. Even assuming the SUV carries a passenger plus the driver, it’s still contributing twice as much to global warming per person.

The final numbers are likely even more lopsided. Given that a typical SUV weighs about 20 times what a scooter does, it’s a safe bet manufacturing an SUV uses way more resources. While it’s tough digging up numbers for a good apples-to-apples comparison, the estimates I can find suggest car manufacturing produces anywhere from 12 to 65 tons of greenhouse gases per vehicle, whereas building a scooter may produce less than 5.

So what’s the takeaway here? Scooters emit more pollution, but they help the planet overall by adding less to greenhouse gas buildup. Or, to put the matter in more downbeat terms, your choice is between choking in the city or dying from mass climate change.

But let’s not be defeatist. Any way you slice it, scooters are less resource-intensive than cars. Less stuff goes into making them. They take less energy to operate. They’re smaller and lighter than cars, so you can crowd a lot more of them onto the streets or into parking lots. They tear up the roads less. OK, maybe in a northern climate scooters aren’t so practical for year-round use, and they can’t haul much cargo, so they’re not going to replace cars altogether. But with resources becoming scarcer, we’re heading for a more densely urbanized future, and little vehicles make more sense than big ones. Once the infrastructure’s in place, a scooter would make the perfect plug-in electric, which solves the pollution problem.

So don’t give your coworker too much grief. You’re looking at the shape of things to come.

Questions we’re still thinking about

Dear Cecil:

Is there any treatment for the snail invasion at the riverside? Have any people died from it? Would not drying your clothes for an extended time kill some of the bacteria instead of letting them air dry?

— carmelarachelle

Dear Cecil:

I am on the Veterinary Faculty of Istanbul University. My department is Reproduction and I am specialised in poultry. I am recently preparing a project about ostrich semen. Unfortunately we have neither knowledge nor any document about this subject. I will appreciate so much if you could give me some information. Thanks in advance.

— Dr. S— A—, Istanbul

Dear Cecil:

If the pink grapefruit sitting in my fruit bowl spontaneously turned into a grapefruit-sized sun, what would happen to my flat, London, and the rest of the world? If I put it somewhere safe, could I enjoy not paying for central heating? Or would it end life as we know it by melting through my floor, into the African textile shop, through the subway system, and finally to a fiery chasm in the middle of the earth where it would make all volcanoes erupt and kill everything, before coming out the other side and changing the way all the planets spin? Thank you.

— Sophie

Cecil replies:

One thing at a time, Soph. You got an ostrich?

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.