Most of the time when I first turn on the A/C in our 2008 Toyota Sienna, the air that blows out has a very distinct chemical - nasty smell. After a while it seems to go away. I'm not sure if I just get used to the smell or it actually goes away. When I just turn on the fan with no A/C there is no smell. Also if I drive somewhere then leave shortly thereafter, it doesn't stink when I fire it up the 2nd (or subsequent) time. Should I be concerned for my health or safety?

  • 1
    Can you give more information on the smell? Is it a mildew smell (think dirty socks)? If not, it could be engine coolant or A/C refrigerant.
    – S_Niles
    Commented Mar 8, 2011 at 18:49
  • I think it's probably mildew. I guess I'm not good at describing smells but that sound right.
    – polarbear
    Commented Mar 9, 2011 at 8:26
  • It smell like when you have a refrigerator closed for couple days .closed then you open
    – user3094
    Commented Apr 30, 2013 at 22:58
  • You also should refer to related questions here and to the right side of the page: mechanics.stackexchange.com/q/2056/57 and mechanics.stackexchange.com/q/2165/57 are definitely relevant.
    – Bob Cross
    Commented May 1, 2013 at 11:38

6 Answers 6


This is a common problem for all air conditioners (in a car or not), and is caused by mildew growth. In cars it often happens when people run their A/C on the recirculation all of the time, or the drain gets clogged. The system doesn't dry out completely and mildew starts to grow.

You should be concerned about your health, especially if you have allergies. Just imagine all that mildew and god knows what else growing in there and being spewed in your face every time you turn the A/C on... Here's a link to US EPA page describing how mold may affect health, if you are still not convinced.

The things you should do to remove the cause of your problem and prevent it from happening again:

  • Run it on recirculation only when something stinks outside, or you want it to cool down quickly. Fresh air from outside will help it dry out better.

  • Make sure that your A/C drain isn't clogged and there is no water building up.

And this is what you could do to remove the unpleasant effects:

  • Run the heater on full for a while, that will dry out the system and might 'cook' the mildew.

  • Change your cabin air filter (if you have one).

  • There are special sprays sold to remove the mildew from the A/C system (read the instructions carefully before using them). Just using Lysol or some other stuff like that will work too, but the smell will be more unpleasant.

I suggest that you do all of this, and in the specified order.

  • 3
    In HVAC this is known as stinky sock syndrome...
    – Jacob
    Commented Feb 27, 2014 at 21:10
  • 7
    Why would outside air help dry it? Also, what is "it", the air inside? I imagine the outside air is much more humid. Circulated air is more preferred because it makes the interior dryer. So I must be thinking about this the wrong way...
    – Bort
    Commented Jun 28, 2015 at 19:45
  • @Bort it largely depends on where you live. Here in Arizona, outside air is most definitely less humid than inside air.
    – Hellreaver
    Commented Nov 4, 2015 at 19:23
  • 1
    Follow-up question: mechanics.stackexchange.com/q/26762/15334 (Which knob setting best cooks the mold?)
    – MWB
    Commented Mar 2, 2016 at 2:30
  • 1
    If you live in a humid area, leaving the car in normal (not recirculation mode) is definitely a BAD thing to do. Yes, you need to 'change' the air occasionally, but remember the mold and mildew is caused by water inside of the HVAC system. You also need to check that the drain is clear, and that there are no leaves or dirt inside the system that can trap water.
    – zipzit
    Commented Apr 26, 2016 at 4:17

I have observed this - kind of like an acrid or "vinegar" smell.

You can help prevent this by turning OFF the air conditioner a couple/few minutes prior to turning off the auto.

As others said, it is caused by growth of biological bacteria/fungal and turning off early helps dry the system and assists in prevention as it reduces the moisture retained in the system.


A possible solution to your a/c smell is to change the pollen filter as they tends to clog after some time. It should be changed annually, hope this helps.

  • Clogged air filter would smell the same, AC or no AC. Commented Feb 9, 2016 at 13:30
  • @DmitryGrigoryev Pollen filter, not air filter. It can accumulate moisture. Commented Feb 18, 2016 at 10:02
  • I stand corrected, thanks. The rest of the comment won't change though: pollen filter goes before AC and can't get moisture from it, so it would smell the same whether you turn AC on or off. Commented Feb 18, 2016 at 10:11

The simplest/easiest/quickest way is to just to turn on the heater full blast for about 5 minutes. This will dry out the air conditioner and kill the mold and bacteria.


Remove the cabin air filter first and start the engine and A/C and select recirculate, take a can of Oziom odor and bacteria killer and spray a good amount into the system where you removed the filter. Make sure to replace that filter ever so often to prevent that from happening again.


First, it's fungus that's growing in all a/c. You can remove it yourself by using an anti-fungal spray at a fitment centre. Spray it on the outside where the wind gets into the car. When you turn your interior fan on there's a suction entery place where the air goes enters your engine bay, spray it there. It will get sucked into the pipes straight to where the fungus is.

  • @Chenmunka, I'm curious about that (seen a couple of similar comments in other SE sites) - is there a place where StackExchange mentions English specifically?
    – Zabba
    Commented Jul 2, 2016 at 6:29
  • @Zabba - I know this is a bit late, but here is SO's Official Non-English Question Policy. Commented Sep 30, 2016 at 0:31

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