How much isopropyl alchcol to deodorize AC ducts?

Question

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:

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).