# Why is the top of a ceiling fan blade dirtier than the bottom?

**URL:** <https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464>\
**Category:** Factual Questions\
**Created:** [July 1, 2011, 2:25am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464 "2011-07-01T02:25:26Z")\
**Posts on this page:** 14\
**Page:** 1

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**Author:** ![JBDivmstr](https://avatars.discourse-cdn.com/v4/letter/j/b4bc9f/32.png) [@JBDivmstr](https://boards.straightdope.com/u/JBDivmstr)\
**Post date:** [July 1, 2011, 2:25am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/1 "2011-07-01T02:25:26Z")

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Pretty much, just like it says…  
The times that I’ve cleaned off the blades of a ceiling fan, the top is usually _ **much** _ dirtier than the bottom. (Said fan blowing the air _ **down** _, the majority (\<75%) of the time.) Why does that happen?  
I understand that the blades tend to attract dust, due to static electrical “cling”, but I’m just wondering why the _ **top** _ of the blade seems to collect a significantly _ **greater amount** _, than the _ **bottom** _. 😕  
I appeal to the greater wisdom of the SDMB, to enlighten me!😃

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**Author:** ![Peremensoe](https://avatars.discourse-cdn.com/v4/letter/p/ce7236/32.png) [@Peremensoe](https://boards.straightdope.com/u/Peremensoe)\
**Post date:** [July 1, 2011, 2:27am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/2 "2011-07-01T02:27:46Z")

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Gravity. The blades aren’t spinning all the time, and some of what falls on the top of the blades sticks.

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**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\
**Post date:** [July 1, 2011, 2:30am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/3 "2011-07-01T02:30:16Z")

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Perhaps the dirt that hits it isn’t getting ‘blown’ off since it’s angled away from the direction the fan it turning. You could try running the fan in the other direction for a few months and see if that changes anything.

My other idea would be simply because that’s where the dust settles. Maybe the dust doesn’t stick to it while it’s moving but after you turn it off. Just like any other piece of furniture, you don’t find nearly as much dust on the bottom surfaces as you do on the top.

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**Author:** ![Cub\_Mistress](https://avatars.discourse-cdn.com/v4/letter/c/ea666f/32.png) [@Cub\_Mistress](https://boards.straightdope.com/u/Cub_Mistress)\
**Post date:** [July 1, 2011, 2:40am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/4 "2011-07-01T02:40:02Z")

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Dust settles by gravity on the top even if the blades are continuously spinning. I have fans I only turn off to dust and they always have dust on the tops of the blades as well as the leading edges.

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**Author:** ![TimeWinder](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@TimeWinder](https://boards.straightdope.com/u/TimeWinder)\
**Post date:** [July 1, 2011, 2:40am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/5 "2011-07-01T02:40:20Z")

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**spark240** got it in one word. Same reason nobody dusts the underside of a shelf.

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**Author:** ![JBDivmstr](https://avatars.discourse-cdn.com/v4/letter/j/b4bc9f/32.png) [@JBDivmstr](https://boards.straightdope.com/u/JBDivmstr)\
**Post date:** [July 1, 2011, 2:40am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/6 "2011-07-01T02:40:40Z")

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> [@spark240](#):
>
> Gravity. The blades aren’t spinning all the time, and some of what falls on the top of the blades sticks.

Gravity? :dubious:  
For some reason, that just doesn’t sound quite right. For one thing, I’ve noticed the phenomenon occurs with ceiling fans that are almost _ **never** _ turned off.  
The same thing also happens to “floor” fans, that are blowing air, _ **horizontally** _. 🙂

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**Author:** ![JBDivmstr](https://avatars.discourse-cdn.com/v4/letter/j/b4bc9f/32.png) [@JBDivmstr](https://boards.straightdope.com/u/JBDivmstr)\
**Post date:** [July 1, 2011, 2:43am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/7 "2011-07-01T02:43:45Z")

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> [@TimeWinder](#):
>
> **spark240** got it in one word. Same reason nobody dusts the underside of a shelf.

Well then, if gravity is the cause… How does that explain the dust on the **underside** of a shelf? 😛

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**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\
**Post date:** [July 1, 2011, 2:49am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/8 "2011-07-01T02:49:25Z")

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> [@Joey\_P](#):
>
> Perhaps the dirt that hits it isn’t getting ‘blown’ off since it’s angled away from the direction the fan it turning. You could try running the fan in the other direction for a few months and see if that changes anything.
> 
> My other idea would be simply because that’s where the dust settles. Maybe the dust doesn’t stick to it while it’s moving but after you turn it off. Just like any other piece of furniture, you don’t find nearly as much dust on the bottom surfaces as you do on the top.

There’s a thin layer of air around the fan that doesn’t move (called “the boundary layer”) - so dust doesn’t get blown off the top because for small enough dust particles, there is no wind to do the blowing

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**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\
**Post date:** [July 1, 2011, 2:49am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/9 "2011-07-01T02:49:41Z")

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Gravity (IMO) is not an entirely sufficient explanation as there will typically be more dust on the front surface of a box fan blade (standing vertically) than the rear surface.

Some thoughts by other people

> <https://physics.stackexchange.com/questions/1343/why-does-dust-stick-to-rotating-fan-propeller>

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**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\
**Post date:** [July 1, 2011, 2:51am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/10 "2011-07-01T02:51:14Z")

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Two reasons: Gravity as mentioned for when the fan is not blowing. When the fan is blowing, there’s gravity, plus the air is flowing from the top to the bottom (when the fan is blowing down as you say it is 75% of the time). That means any dust in the air that impacts the fan blades due to the air flow is going to impact the top side 75% of the time.

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**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\
**Post date:** [July 1, 2011, 3:18am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/11 "2011-07-01T03:18:43Z")

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> [@OldGuy](#):
>
> …the air is flowing from the top to the bottom…That means any dust in the air that impacts the fan blades due to the air flow is going to impact the top side 75% of the time.

Right. A fan blade, horizontal or vertical, creates a breeze and is in the path of that breeze. The air in said breeze has dust in it. The dust is being thrust onto the fan blades. Of course it’s going to accumulate on the side it hits, and not on the side it doesn’t hit.

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**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\
**Post date:** [July 1, 2011, 4:09am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/12 "2011-07-01T04:09:34Z")

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dust will collect on the low pressure side of a moving fan blade and motor and supports that are in the airstream on the low pressure side of the unit. dust will collect on the top of nonmoving things as well.

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**Author:** ![Autolycus](https://avatars.discourse-cdn.com/v4/letter/a/22d042/32.png) [@Autolycus](https://boards.straightdope.com/u/Autolycus)\
**Post date:** [July 1, 2011, 6:53am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/13 "2011-07-01T06:53:27Z")

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The dust that settles on the top part of the fan blade is actually how they kill you in your sleep.

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**Author:** ![automagic](https://avatars.discourse-cdn.com/v4/letter/a/a698b9/32.png) [@automagic](https://boards.straightdope.com/u/automagic)\
**Post date:** [July 1, 2011, 7:02am UTC](https://boards.straightdope.com/t/why-is-the-top-of-a-ceiling-fan-blade-dirtier-than-the-bottom/587464/14 "2011-07-01T07:02:48Z")

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Maybe fan blades need spoilers 😛