# Exhaust from Car X is cleaner than the air in City Y.
**URL:** https://boards.straightdope.com/t/exhaust-from-car-x-is-cleaner-than-the-air-in-city-y/591410
**Category:** Factual Questions
**Created:** [August 4, 2011, 2:29am UTC](https://boards.straightdope.com/t/exhaust-from-car-x-is-cleaner-than-the-air-in-city-y/591410 "2011-08-04T02:29:28Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@automagic](https://boards.straightdope.com/u/automagic)
#### Post date: [August 4, 2011, 2:29am UTC](https://boards.straightdope.com/t/exhaust-from-car-x-is-cleaner-than-the-air-in-city-y/591410/1 "2011-08-04T02:29:28Z")
I have heard from several different sources that some cars have such clean engines and such efficient catalytic converters that the exhaust they produce is cleaner than the air in some cities, like Los Angeles. Furthermore they say that if Car X was in the very dirty air of City Y it would literally be cleaning the air of the city since its exhaust is cleaner.
To me that seems illogical. Even if the car is super efficient and clean it will still produce some pollution. The exhaust of the car will be the dirty air of the city plus the pollution the car created. The only way I can imagine a car cleaning dirty air is if the air had Hydro-Carbons, Oxides of Nitrogen or Carbon Monoxide since the catalytic converter would handle that. The problem is that, that is true for any car with a Three Way Cat.
Is there any truth to what I am saying or am I wrong?
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### Author:  [@DorkVader](https://boards.straightdope.com/u/DorkVader)
#### Post date: [August 4, 2011, 2:43am UTC](https://boards.straightdope.com/t/exhaust-from-car-x-is-cleaner-than-the-air-in-city-y/591410/2 "2011-08-04T02:43:34Z")
I remember reading something like this years ago(late 80s early 90s), trying to find something I can link to now, but, google isn’t my strong point.
IIRC this was debunked because they were only counting city air samples taken at certain times of day (peak traffic hours i think) or something like that and not evaluating city air overall through out a 24 hour period or something to that effect.
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### Author:  [@Rick](https://boards.straightdope.com/u/Rick)
#### Post date: [August 4, 2011, 2:52am UTC](https://boards.straightdope.com/t/exhaust-from-car-x-is-cleaner-than-the-air-in-city-y/591410/3 "2011-08-04T02:52:38Z")
You are incorrect.
Two data points. Back in 1990, I was a smog test technician at a shop located right next to the San Diego Freeway in West Los Angeles. I had 8 lanes of traffic right outside the back of my building (had a car go off the side the freeway once and wind up in our lot)
When doing the test (BAR-90) the system would sense the air in the shop, and then the probe would be placed in the tailpipe for the actual test.
I regularly saw test results with negative hydrocarbon readings. The lowest I think I ever saw was -12PPM
C0 readings would be zero or extremely low
BAR-90 was not a dyno test so no measurement of NOX was done.
A thing to note this was on fuel injection systems that were then current but are now about 5 generations old.
Second data point, when Volvo came out with their PZEV engine (Partial Zero Emission Vehicle) The instrumented a couple of them and drove around LA with officials from the State of California. They had sensors on the front of the car measuring HC, CO NOX, and ozone.
Since the cars had ozone cleaning radiators, they also sampled the air behind the radiator for ozone.
The test results? Yup the car were cleaning the air while being driven. I wasn’t privy to the numbers, but they were some negatives in there.
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### Author:  [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)
#### Post date: [August 4, 2011, 3:04am UTC](https://boards.straightdope.com/t/exhaust-from-car-x-is-cleaner-than-the-air-in-city-y/591410/4 "2011-08-04T03:04:22Z")
> [@automagic](#):
>
> The only way I can imagine a car cleaning dirty air is if the air had Hydro-Carbons, Oxides of Nitrogen or Carbon Monoxide since the catalytic converter would handle that.
That’s mostly what they mean when they talk about a car running “clean”. I’m not sure if anyone has made the claim that newer cars clean up other air pollutants not usually associated with motor vehicles (other than the ozone thing).
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### Author:  [@automagic](https://boards.straightdope.com/u/automagic)
#### Post date: [August 4, 2011, 3:54am UTC](https://boards.straightdope.com/t/exhaust-from-car-x-is-cleaner-than-the-air-in-city-y/591410/5 "2011-08-04T03:54:06Z")
> [@Rick](#):
>
> You are incorrect.
> Two data points. Back in 1990, I was a smog test technician at a shop located right next to the San Diego Freeway in West Los Angeles.
I am going to hi-jack my own thread.
**Rick** I would like to know where I can find information on how to become a smog tech. I have my A1-A8 ASE’s and I will be taking and (hopefully passing) my L1 in November. What else do I have to do? Also any advice?
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### Author:  [@DorkVader](https://boards.straightdope.com/u/DorkVader)
#### Post date: [August 4, 2011, 4:58am UTC](https://boards.straightdope.com/t/exhaust-from-car-x-is-cleaner-than-the-air-in-city-y/591410/6 "2011-08-04T04:58:03Z")
> [@Rick](#):
>
> You are incorrect.
> Two data points. Back in 1990, I was a smog test technician at a shop located right next to the San Diego Freeway in West Los Angeles. I had 8 lanes of traffic right outside the back of my building (had a car go off the side the freeway once and wind up in our lot)
> When doing the test (BAR-90) the system would sense the air in the shop, and then the probe would be placed in the tailpipe for the actual test.
> I regularly saw test results with negative hydrocarbon readings. The lowest I think I ever saw was -12PPM
> C0 readings would be zero or extremely low
> BAR-90 was not a dyno test so no measurement of NOX was done.
> A thing to note this was on fuel injection systems that were then current but are now about 5 generations old.
> Second data point, when Volvo came out with their PZEV engine (Partial Zero Emission Vehicle) The instrumented a couple of them and drove around LA with officials from the State of California. They had sensors on the front of the car measuring HC, CO NOX, and ozone.
> Since the cars had ozone cleaning radiators, they also sampled the air behind the radiator for ozone.
> The test results? Yup the car were cleaning the air while being driven. I wasn’t privy to the numbers, but they were some negatives in there.
I’m not gonna argue, can’t find the story I thought I remembered anyway.
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### Author:  [@IndyDMan](https://boards.straightdope.com/u/IndyDMan)
#### Post date: [September 12, 2019, 3:19pm UTC](https://boards.straightdope.com/t/exhaust-from-car-x-is-cleaner-than-the-air-in-city-y/591410/7 "2019-09-12T15:19:26Z")
An Edmunds test done back in 2011 was about how modern light truck exhaust compared to lawn equipment.
Here is the Edmund article:
> **[Emissions Test: Car vs. Truck vs. Leaf Blower | Edmunds](https://www.edmunds.com/car-reviews/features/emissions-test-car-vs-truck-vs-leaf-blower.html)**
>
> Edmunds.com conducts an Emissions Test of the 2011 Ford F-150 SVT Raptor Crew Cab, 2012 Fiat 500 and Leaf Blowers.
If you just got a F150 on the highway and up to speed you’d have to drive it 3,887 miles before you equaled the non-methane hydrocarbon exhaust pollution of the 2-cycle, backpack leaf blower running for 30 minutes - half an hour.
There are guys out there on lawn crews running those things four or five hours a day! Put on a mask!!! A really good one.
Trucks and cars are probably better now. Don’t know where they’ll be in X years.
I’ve been to China a lot. In the winter especially, when they get temperature inversion, they could use fleets of 150s rolling through their cities streets. I was near Fuzhou one December and the \<10PPM particulates were at 660 per square meter. The US FDA scale, I hear, only goes up to 500. It looks like fog, only it burns. We were going that way in the 60s. Clean Air Act put us where we are now: breathin’ easier.