# Why are water filters supposed to be kept submerged?

**URL:** <https://boards.straightdope.com/t/why-are-water-filters-supposed-to-be-kept-submerged/409837>\
**Category:** Factual Questions\
**Created:** [June 28, 2007, 10:47pm UTC](https://boards.straightdope.com/t/why-are-water-filters-supposed-to-be-kept-submerged/409837 "2007-06-28T22:47:42Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![misterW](https://avatars.discourse-cdn.com/v4/letter/m/f1d935/32.png) [@misterW](https://boards.straightdope.com/u/misterW)\
**Post date:** [June 28, 2007, 10:47pm UTC](https://boards.straightdope.com/t/why-are-water-filters-supposed-to-be-kept-submerged/409837/1 "2007-06-28T22:47:42Z")

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I’m talking about the carbon brita-type filters. They usually have a line on the pitcher and you are supposed to keep the water level above that line. Why is that? I believe the filters work by bonding with impurities in the water…but I dont see how the filter’s effectiveness would be hurt by letting the water drop below the line. Would carbon particles possibly come out of the filter when you use it (like when the filter is first used)if it is allowed to dry?

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**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\
**Post date:** [June 29, 2007, 12:06am UTC](https://boards.straightdope.com/t/why-are-water-filters-supposed-to-be-kept-submerged/409837/2 "2007-06-29T00:06:39Z")

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[Brita Faqs](http://www.brita.com.au/help/frequently_asked_questions)

> [@](#):
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> Although the cartridges are packed wet they can occasionally dry out during storage and transportation. Abrasion may occur and a fine carbon dust can form which is washed out during the first two fills. This water is safe but unatractive, therefore we recommend that the water from the first two fills be discarded or used to water flowers and plants etc

Also, activated carbon can be pretty water repellent. That means that it may take a long soak to wet/hydrate the material sufficiently for it to be an effective filter.

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**Author:** ![boytyperanma](https://avatars.discourse-cdn.com/v4/letter/b/4491bb/32.png) [@boytyperanma](https://boards.straightdope.com/u/boytyperanma)\
**Post date:** [June 29, 2007, 2:44am UTC](https://boards.straightdope.com/t/why-are-water-filters-supposed-to-be-kept-submerged/409837/3 "2007-06-29T02:44:43Z")

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Yes you are likely to get carbon particles if you let it dry out then run water through it again. Also activated carbon is a breading ground for bacteria if left unchecked. Many bacterial will grow faster in a moist environment than a wet one.