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i'm looking to remove an odor from my Corolla's fresh air intake.

There are products available where you spray the product into the fresh air intake (near the windshield wipers), while the fan is running full speed:

enter image description here

Toyota has a blurb about how to perform something similar yourself:

What causes air conditioner odor? How can I prevent the odor from occurring?

... spraying a mixture of isopropyl alcohol and water (1 to 5 ratio/mixture) or a disinfectant in the outside air intake may help reduce the smell.

Archived copy:

What causes air conditioner odor? How can I prevent the odor from occurring?

During air conditioner operation, cold refrigerant is pumped through the evaporator core by an engine-driven compressor. A fan then blows air through "fins" in the evaporator to cool the air. These fins also act as an air filter, trapping bacteria, spores, and dirt. These airborne particles are normally washed out a drain hole with condensation, but if they remain on a moist evaporator, they may collect and cause an unpleasant odor. This effect is more frequently found in humid climates where more condensate accumulates. This situation is not unique to Toyota; it is an industry-wide condition.

To prevent the odor, Toyota recommends the following:

  • Avoid parking under trees to reduce the possibility of leaves entering the air intake
  • Use the fresh air setting on your climate control rather than the recirculated air setting whenever possible to allow the evaporator to dry out
  • Drive on paved roads whenever feasible as dusty conditions may accelerate the condition
  • If the condition already exists, spraying a mixture of isopropyl alcohol and water (1 to 5 ratio/mixture) or a disinfectant in the outside air intake may help reduce the smell. If these steps do not alleviate the odor, we encourage you to contact your local dealer for a thorough evaluation of the condition.

Now the idea of spraying an aerosol into my car's electric fan makes me kind of nervous:

  • the product in the spray can is (almost certainly) flammable and explosive - yet Toyota itself uses the same thing
  • i've also seen videos of people spraying Lysol into their fresh air intake to remove odors. A can of Lysol can double as a flame-thrower for those with too much time on their hands
  • i also bought a can of AC deodorizer that contains iso-butanol; and has flammable warnings on the can
  • and finally we have Toyota recommending spraying isopropyl alcohol (i.e. rubbing alcohol) into your intake

i'm willing to try the isopropyl alcohol, but i'm confused about the ratio. They recommend 5:1 ratio. Is that:

  • 5 parts alcohol, 1 part: 83% alcohol, or
  • 5 parts water, 1 part alcohol: 16% alcohol

Again, their wording:

a mixture of isopropyl alcohol and water (1 to 5 ratio/mixture)

i came across a forum post where a person said something to the effect of:

i already know about using isopropyl alcohol (5 parts alcohol with 1 part water)

Which is the opposite of how i read Toyota's instructions.

i did some tests:

  • 91% isopropyl alcohol: flammable (9:1)
  • 70% isopropyl alcohol: flammable (7:1)
  • 50% isopropyl alcohol: not flammable (1:1)
  • 13% isopropyl alcohol: not flammable (1:5)

13% isopropyl alcohol is very watery - with a lot of water left over (it doesn't evaporate very well).

So my question:

How much isopropyl alchcol to deodorize AC ducts?

Or am i being too cautious?

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The Toyota links are dead. What I could Google no longer recommends any DIY solution. – MaxB Feb 25 at 21:02
up vote 3 down vote accepted

a mixture of isopropyl alcohol and water (1 to 5 ratio/mixture)

means 1 measure of alcohol to 5 of water. You wouldn't want to go the other way, in my opinion as you'll end up with a lot of alcohol, which will not only be a fire risk, but also could do bad things to duct work etc.

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i ended up using, after extensive testing, a 50-50 mixture.

Turn fan on full-blast, and use spray-bottle to spray mist into the intake.

Odor eliminated; i was surprised it worked.

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What volume of the 50/50 mixture would you say you ended up using ? Did you remove the A/C filer while doing so ? – Bertrand Caron Jan 27 at 22:07
    
@BertrandCaron It was difficult to use an old Windex bottle to spray stuff through the tiny vent openings below the windshield. I didn't remove the cabin air filter. But after recently replacing my blower motor, and seeing a lot more of how things move, i would take out the filter, and spray directly into the place where the filter does - directly into the fan blower. But i sprayed as much as i could stand; about a quarter of a windex bottle. – Ian Boyd Jan 28 at 15:15
    
Curious to know how long that lasted before the odor came back. – MaxB Feb 25 at 21:03
    
@MaxB Only about two months. – Ian Boyd Feb 25 at 21:58

I thought I'd post a warning here. According to Wikipedia,

https://en.wikipedia.org/wiki/Isopropyl_alcohol

Isopropyl alcohol has a 2-12% explosive limit, meaning if the concentration of it in the air reaches above 2% (by evaporating from the mixture and concentrating somewhere, as it's heavier than air), it can explode.

This is different from trying to set its solution on fire.

Perhaps this method should be avoided, depending on your risk tolerance.

Additionally, isopropyl alcohol attacks some plastics and aluminum, apparently.

Another suggested approach is to try to cook the mold:

Which knob setting best cooks the mold inside HVAC?

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It should be noted that the Explosive Limit is a concentration of pure isopropyl in the air - having 2% of the air contain alcohol. In this case we are diluted, and running a fan. The concentration doesn't actually get very high. The amount of alcohol would have to exceed 20mL per litre of air. The fan on high moves maybe 3-4L per second. And i certainly cannot spray 20 mL from my spray bottle in 1 minute, let alone 200ms. – Ian Boyd Mar 2 at 3:41
    
@IanBoyd The amount of alcohol would have to exceed 20mL per litre of air Sorry, but your math is VERY, VERY wrong. You are confusing the volume of liquid matter with its volume as a gas. You are also confusing concentration and volume ratios. – MaxB Mar 2 at 4:57
    
@IanBoyd also, the point of spraying isopropyl into HVAC is that it will accumulate somewhere long enough to kill mold. If it was getting removed right away, as you suggest, it just wouldn't have any effect, so clearly, it isn't. – MaxB Mar 2 at 4:58

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