Types of Pollutants
There are two types of pollutants that come out of an exhaust pipe. There is carbon dioxide, (CO2), yes the climate change stuff, and then all the other little nasties which actually cause health problems from smog. Carbon dioxide is always produced when you burn gas because that’s what burning gas is. Carbon in gas is combined with two oxygen’s from the air and you get CO2. No other way to do it.
The other group of bad stuff isn’t fundamentally a part of the process but it’s very hard to make zero. When gas (carbon) doesn’t get quite enough air, the carbon teams up with just one oxygen instead of two and you get carbon monoxide (CO) which sounds similar to CO2 but it’s much worse because it kills you by stopping your lungs ability to absorb oxygen (this is how people commit suicide in their garage). So a little carbon monoxide is produced as are other bad stuff like nitrous oxides (NO & NO2). These nitrous oxides cause smog and cancer. There’s a few other nasties too but you get the idea.
Figuring out carbon (aka carbon dioxide or CO2) emissions is easy because you just have to look at how much fuel you’re burning. If a scooter burns less gas then a car then it’s producing less carbon than a car. Simple. So virtually every scooter produces less carbon than even the best cars. The biggest maxi scooters tend to be about on par with fuel efficient hybrids, as they both achieve about 45 mpg and thus produce similar carbon. Thus the claim that scooters are worse is dead in its tracks if you’re referring to carbon pollution and climate change.
This is where things get more complicated and it’s where the claim that scooters are worse arises from. Cars have all sorts of fancy systems to avoid producing these other bad gases like double-burning the exhaust and catalytic converters which switch these bad gases into less bad gases. Some scooters have some of these systems but scooter motors are a simpler, smaller and cheaper so they can’t easily have all of these cool controls and still cost $1500. Some do try though, such as Honda’s Ruckus, which has system that adds extra air into the exhaust to try to keep the gases burning for longer to reduce the carbon monoxide. Starting in 2006, Honda also added a catalytic converter to the Ruckus, so the Ruckus actually produces pretty darn low emissions. Still, if you burned 10 gallons of gas in a Ruckus it would produce more little nasties than that same 10 gallons burned in a modern car.
This is where things get a bit tricky because scooters do burn less clean (“higher concentration of nasties”) but they also burn a lot less fuel. This important differences doesn’t show up in emissions results, because emissions testing just measuring the concentration of nasties (e.g. parts per million) and not the total amount. For the average scooter it will burn about 1/2 as much gas at about 2x the concentration of emissions, so total emissions are similar.
A complexity here is that the legal limits for scooters are much higher and many people assume that scooters must pollute near the limit, when most do quite a bit better. Scooters are typically allowed to pollute 4x as much carbon monoxide and there’s no limit on nitrous oxides, but most modern scooters don’t pollute anywhere near this. Typically the fuel injected scooters that achieve outstanding milage also achieve outstanding emissions since burning gas efficiently is good for both.
But Sometimes They’re Right (2-Strokes)
As discussed, you need to look at both the pollution concentration and exhaust volume to figure out your total emissions of nasties. In most scooters, the much lower exhaust volume more than makes up for a somewhat elevated concentration of nasties. However, some scooters put out ridiculously high concentrations of pollutants so even low exhaust volumes can’t save them. I’m mostly referring to 2-strokes here, but I’m sure there are some horribly out of tune 80’s 4-strokes with sky high levels also.
Unlike 4-strokes, 2-stroke motors inject oil in with the gas and both get burnt. With oil in the gas, it’s tough to burn away all the bad stuff. Also, 2-strokes are less efficient because they do a rushed job of burning gas. Thus, most 2-strokes - and especially old ones - produce pretty darn high emissions and are truly worse than cars for many pollutants (but they’re still lower for carbon of course). Most modern 2-strokes have additional controls to try to bring the pollution levels down (e.g. the heavily restricted 2-stroke Zuma that returned for 2008). These modern 2-strokes are less bad, but the emissions are still usually higher and the control systems are commonly disabled as people swap exhausts and toss on big bore cylinders, so a good portion of modern 2-strokes still pollute quite heavily.
To wrap this up, all scooters produce less carbon pollution than cars because they burn less gas. Most scooters do produce slightly higher concentrations of other pollutants (mostly carbon monoxide and nitrous oxides that cause smog), but because they produce much less exhaust the total release for these pollutants is also lower. The main group of scooters that can be argued as polluting worse than cars are vintage and modified 2-strokes, which are still low carbon emitters but can release pretty ridiculous levels of other pollutants. However, 2-stroke scooters are becoming increasingly rare and it’s not fair to lump the much better modern 4-stroke scooters in the same boat. It can only take one blue-smoked Lambretta to sway a car drivers opinion, but the reality is that most scooters are much easier on the planet.