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Discussion Starter · #1 ·
The original thread contained a mostly jocular repartee on the ability of the Prius to clean the air.

This post runs the hard numbers on carbon monoxide.
I think we will all be surprised.

First we will consider the Prius on idle in the enclosed garage.
Second we will examine the undiluted exhaust.

For reference, we need the toxicity of carbon monoxide (hereafter CO).

The EPA describes >15 ppm CO as poor air quality.
The highest CO level recorded in the US (was in Yellowstone Park) in 1996 was 36 ppm.

From the MSDS:
200 ppm: Slight symptoms (headache, discomfort) after several hours of exposure.
400 ppm: Headache and discomfort experienced within 2-3 hours of exposure.
1,000 -2000 ppm: Within 30 minutes, slight palpitations of the heart occur. Within 1.5 hours, there is a tendency to stagger. Within 2 hours, there is mental confusion, headache, and nausea.
2000-2500 ppm: Unconsciousness within 30 minutes.
>2500 ppm: Potential for collapse and death before warning symptoms are produced.

[Here I will use ppm by weight. CO has the same molecular weight as N2, so ppm by volume is not significantly different.]
1. Prius on idle in enclosed garage:
--The Prius is SULEV. Take the SULEV standard for carbon monoxide of less than 1 g/mile.
--The Prius EPA mileage is 48 mpg.
--To yield 48 g CO per gallon of gasoline consumed.
--The Prius tank is 11.9 gallons.
--The Prius will idle for about ten days or about 1.2 gallons per day.
--To yield 58 g CO after idling for 24 hours.

We need a garage.
Take a two-car garage of 20 X 20 X 10 ft for 4000 ft3 or 113 m3.
Air weighs 1.275 kg/m^3 for 144 kg (144,000 g) of air.
From above, 24 h of idle yielded 58 g CO.
Thus, 400 ppm (58/144000) CO are produced after idling the Prius for 24 hours in a 2-car garage.

If we spend 2-3 hours in an enclosed garage in which the Prius has been idling for 24 h, we will get a headache.

Practical Application: While not advised, <1 h cabin warmup on idle in an enclosed garage should not pose major health concerns. After 1 h, roughly 17 ppm of CO will be released. This is just above the EPA threshold of poor air quality (>15 ppm) and less than half of the maximum 1996 US value (36 ppm).

2. Undiluted Prius Exhaust:
--From above 48 g CO produced per gallon of gas used.
--Gas weighs 6.1 lbs per gallon.
--A stoichiometric burn is 14.7 lbs air to 1 lb gas.
--That is, a pound of gas produces 15.7 pounds of exhaust.
--One gallon gas (6.1 lbs) produces 95.8 lbs exhaust.
--Or 48 g CO per 95.8 lbs (43,450 g) exhaust.

--Or 1105 ppm CO in pure Prius exhaust.

So if you breathed undiluted Prius exhaust from car forced to run its gas engine for two hours nonstop you will experience mental confusion, headache, and nausea. Not death.

The Prius will not reduce CO levels in Philly or anywhere in the unenclosed US (1996 max 36 ppm, Prius exhaust 1105 ppm).

Practical Applications (1) after a very, very bad day: Suicide by gas pipe appears impossible using the Prius. Does provide a cooling-off period for the depressed, however. Hopefully, this does not promote driving the wrong way down the freeway as an alternative.

Practical Applications (2): On farms, feral cats are used to reduce vermin. They proliferate and must be culled. A common and humane way to cull is to collect the cats in a burlap sack and tie the sack to the exhaust of a car. The Prius is not the car for this grim, but necessary, task.

Conclusion: There seems little chance that the Prius can generate lethal CO levels.

PS The above is a best faith analysis of Prius exhaust based on published data. No liability is accepted. The Surgeon General has determined that breathing pure Prius exhaust does not improve your sex life.

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Those are interesting calculations, but you forgot an important word in your conclusions, "maximum". There is a maximum of 1105 ppm CO in pure Prius exhaust. It could easily be less. In fact, reported attempts to measure the CO show up zero, which just means it's lower than the test equipment's sensitivity. So we can still hold out hope that our Prius can clean Philly's air. Still, beating the standard by a factor of 33 sounds a pretty unlikely.

Also, gasoline engines produce most of their CO output when changing load or when cold. So the Prius left to idle for 10 days would probably produce more CO than the Prius forced to idle continuously.

Finally, breathing undiluted Prius exhaust would kill you rather quickly even if the Prius were producing no CO at all. Undiluted Prius exhaust contains only trace amounts of O2, not nearly enough to support human life.

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Good Science? The Phili Factor (PF)

The science sounds good .. but, Ever try to idle a Prius for days or even hours? Not me.
Take into account the load on the engine, lights, radio, - runing the AC. There has to be some kind of load to get a prolonged idle. ICE Off/On time will be more frequent in an ice cold garage vs a hot Flordia garage. The Phili factor.

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Actual emission numbers

Everyone understands I was being funny when I suggested that we drive around extra just to clean the air. However, I think the original quote (from Toyota Motor Sales' exec VP) is still valid. (He said in some cities the air coming out of the Prius is often cleaner than the air going in.)

First some actual emission numbers. These are the EPA's SULEV test results for the 2002 Prius, using low-sulfur gas. Note that these results indicate the emissions of a car with 120,000 on its engine (and more importantly, on its catalytic convertor). Format is Emission:result(limit). (I think all units are g/mi.)

For comparison, here are the results for a 2002 Echo (100,000 miles):


CO=carbon monoxide
NMOG=nonmethane organic gas (i.e. aldehydes, ketones, alcohols, ethers, alkanes, alkenes, alkynes, and aromatics) (e.g. stinky stuff)
NOx=oxides of nitrogen

For HCHO, I assume the Prius number is "unmeasurably small". Other cars' EPA test results measured numbers as small as 0.0001, so it is best to say "<0.0001" for the Prius.

For CO, an old Prius will emit 0.34 g/mi (less for a new car). Nathan used 1 g/mi in his calculations, so his results should all be divided by three. This does not significantly change his results.

Rather, the key difference is that the Toyota VP doesn't claim that the Prius' exhaust is cleaner than the city's air. He simply states that down at road level (where concentrations are undoubtedly higher), air out is better than air in. I think this is defendable, and here is my reasoning. Burning enough gas to travel one mile surely creates more than 0.34 g of CO. The exhaust system removes all but 0.34 g. Other cars' exhaust systems are not as good (e.g. Echo). So, the air pulled into the engine contains CO emitted by the car in front of you. The Prius' exhaust system not only removes most of its own CO, but some from the car in front of you. The net effect is still a loss: the air in general is dirtier than if you had stayed home. However, the local effect is a gain: air out is cleaner than air in, so if you have to breathe road-level air in a big city, follow a Prius instead of an Echo (or a Dodge Ram 350: 6.2 g/mi CO!).

Douglas (2002 Silver, Wisconsin)

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Data source

Nathan asked me in a private message where I got my data. Here's the link: . It contains tab-delimited zip files listed by model year. You can open them from Excel and sort/filter to get it down to a manageable size. I couldn't find this raw test data by myself, but I had some long email conversations with an EPA Environmental Scientist named Julia Rege, and she was kind enough to point me to the link.

Nathan was surprised that only CO, HCHO, NMOG, and NOx were listed in the data file for the Prius. He specifically asked about particulate matter (PM), ozone, and hydrocarbons (HC). I was also surprised that only these four categories are listed. I don't know the official answer, but I have made some assumptions from reading a lot of EPA webpages on the subject:

PM: It seems that the PM emissions of gasoline engines are so low that they are not measured--even for SULEV. Conversely, PM emissions of diesel engines are a major problem (compare the emissions of a diesel New Beetle with a gas New Beetle, for example, at ... tle-02.htm ). Apparently the root of the problem is that diesel refined and sold in the US is very dirty compared to that available in Europe. Given the facts, I'm surprised that diesel engines are allowed on US roads.

Ozone: It seems that ozone is not a direct emission of gasoline engines. It is created when NOx reacts with volatile organics (apparently included in the NMOG and HCHO categories) in sunlight.

HC: I think HC is now broken down into HCHO and NMOG sub-categories for Tier 2, but it's just a guess. Other cars are tested for other categories. Ms. Rege explained that our nation has a really nightmarish mess of certification standards, so probably the categories are dependent on the certification level desired by the manufacturer. The important thing is that these four categories are all that matter for SULEV.

You can also see the Dodge Ram 350 results in the detailed data--I chose that one by filtering on CO emissions and sorting from highest to lowest, so it's a worst case.

Another thing Julia explained to me is why the Prius is rated SULEV in California but ULEV everywhere else. It's a question of economics. If you look at the Prius SULEV and ULEV test results, you will see that the emission quantities for both tests are under the SULEV limits, but Toyota chose to certify only to the ULEV level for most of the US, because (1) it's cheaper to run those tests, and (2) they have less liability. You see, ten years down the road, if a lot of Priuses start failing SULEV emissions tests, Toyota might be forced to recall them. But ULEV standards are much slacker, so Toyota knows the Prius will never exceed them, even if manufacturing defects are revealed as the cars age.

One key factor in these test results is sulfur. The EPA performed both SULEV and ULEV tests with low-sulfur gas (30 ppm). The presence of sulfur (US average is 300 ppm) would make the emissions higher (possibly failing SULEV), but I have been unable to find out how much higher. Ms. Rege didn't know of any rules of thumb, but gave me some other links that I've been tracking down in my free time. Since low-sulfur gas is hard to find in most of the US, I want to know what the average person's Prius is emitting. I have been able to find low-sulfur gas in the Milwaukee area, but only in 93 octane. I've just started using it--if I have any problems I'll post them.

Nathan pointed out that SULEV only exists in California's regulations. I consider this to be a technicality. The Prius' emissions have been EPA-certified to be significantly below SULEV limits. It doesn't matter that my state doesn't recognize the standard--the emissions are still SULEV.

Douglas (2002 Silver, Wisconsin)

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Discussion Starter · #7 ·
EPA CO numbers

Since Doug did such a great job finding the emissions data I thought I would update the original post in this thread that used the SULEV CO limit as a maximum for the Prius.

However just getting the tab delimited data is a real nuisance. So for now I will simply restate the EPA values with a few comments. I put the CO g/mile in red. The first column is the state (CA or the rest of the country NL), then something called useful life that I believe corresponds to expected emissions of an older car (mileage in thousands), the test procedure used, CO, the standard (e.g., SULEV=1 g/mi), and the standard name (e.g., sulev).
Sales Area U Life Emission Cert Level Standard Tier
CA 50 CO-COLD 1.8 10
NL 50 CO-COLD 1.7 10
CA 120 CO 0.34 1 SLEV
NL 100 CO 0.18 2.1 ULEV
NL 50 CO 0.12 1.7 ULEV
NL N/A 50-CO 0.4 1.7 ULEV
CA N/A 50-CO 0.4 1 SLEV
CA 4 CO-US06 0.09 8 LEV
NL 4 CO-US06 0.09 8 LEV
CA 4 CO-SC03 0.13 2.7 LEV
CA 4 CO-SC03 0.52 2.7 LEV
NL 4 CO-SC03 0.13 2.7 LEV
NL 4 CO-SC03 0.52 2.7 LEV
Remember that I used <1 g/mi and Doug used 0.34 g/mi (row 3).

Note that the newer (50,000 mile) car, if I understand this right, will emit only 0.12 g/mile (which may help explain Robert's comments).

On the other hand, the cold engine/cold cat gets 1.7-1.8 g/mi for reasons that are explained in the link found in the chemistry of combustion post on the general forum, something I ignored in my ballpark figures as a brief departure from the norm.

The SC03 entries with varying results were otherwise identical in the original and they are included for completeness.
Idle Time Full-Tank
My original calculations depended on an estimate that the full-tank Prius would idle for 10 days (actually 237 hrs) was from Sam Williams here:

Julian estimated 8.5 days (205 hrs) while charging a depleted 12 V battery here:

The full-tank idle time may be strongly dependent on ambient temperature.
I asked Doug about hydrocarbons, but I forgot I used to know the answer as detailed in this neat snippet.
[T]he cleanest motorcycles in the 2001 model year
certified at a level of about 0.32 grams/mile of hydrocarbons on the
Federal Test Procedure. On the exact same test procedure the Prius
certified at a hydrocarbon level of 0.0024 grams/mile. The cleanest
(highway) motorcycle is therefore more than 100 times dirtier than
the Prius.


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PS The above is a best faith analysis of Prius exhaust based on published data. No liability is accepted. The Surgeon General has determined that breathing pure Prius exhaust does not improve your sex life.

Ok, I guess having sex eliminates what to do to pass time while waiting for yourself to croak. Have to hire all of the local call girls before you get your headache. By then order a months supply of Viagra.

Guess better pack a few sandwiches and bring out the cable TV so you don?t die of boredom first.

Maybe wait to write your suicide note and last will and testament while waiting too.

Guess if anyone was successful then they will surly win the "Darwin Award". (Picture Person with dryer hose and duct tape from car straight to his mouth)

Autopsy would show person died of starvation.

Toyota Credit repo man driving away with him still in car because it took so long that he missed his car payments.

Ask Billy Mays to do his commercial on how to use your Prius as a HEPA filter.

Sheesh, what the hell ever got onto this subject anyway.

Just my opinion. :lol:

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If I recall correctly CO is produced as the result of incomplete combustion. Complete combustion produces mainly CO2. As the Prius uses up the oxygen in the garage the combustion process will be more and more incomplete, and more and more CO will be produced. When the oxygen is gone (I'd give it no more than an hour) the ICE should fail to start up within the correct amount of time, and the Hybrid Warning should come on. This would inform the now dead occupant to have his or her Prius checked out at the local dealership. :wink:
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