What are common causes for credit card strips to be demagnetized
(security pass cards, cell phones, store scanners?) and why does it
happen?

Apparently one way is from magnetic clasps on wallets:

http://www.straightdope.com/classics/a3_388.html

hi greetings,

Lets see what the strip contain and how that is organised.

There will be 3 magnetic tracks behind the credit card. Almost all the
card uses only first 2 tracks. Information stored in these tracks
depends upon the bank to bank. (personal details, total outstanding,
PIN...) these information will be stored  in 0's and 1's (bits). One
of these bits length will be long and another short length on the
card. Thats the reason, if you swipe card at different speeds card
reader will read your card.

Let me come to your point. Of course there are many reasons this
strips get damaged.

1. Environment: Place you keep your card, If you are keeping card in
vallet/purse without any cover your material. More humid the climate
more vulnerable the strips. (you might have observed photos in albums
gets faded/stics to album... same case with cards) which will make a
thin layer on magnetic strip, which makes card less readable. This is
the major reason for demagnetization of card.

2. Security pass cards/ Magnetic fields: Strong magnetic field
exposure will change in alignment of information stored on the card
making it invalid.

3. Cell phones/ scanners: will have less effect on the card magent
strip, as, its electromagnetic field will be less.

4. Usage: more you yse the card :-), some tracks get erased due to the
handling at the time of swipe. If the user slightly bends the card
while swiping probability of getting track erased is more.

5. Other: this may be many cases, some other material (most likely
liquids, solids) falls on card stripe.


Hope this will clarify atleast some part of your doubt.

best regards,

Speaking from experience, a common enemy of mag strips is EMF or
Electro Magnetic Field. The EMF has to be either very strong, or, the
proximity of the mag strip very close.

Data is stored on a mag strip much like a cassette tape stores a song,
using magnetic material (like iron oxide) which is arranged in a
precise pattern.  Any exposure to a strong external magnetic field
will scramble the pattern (erase) and render the mag strip useless.
The card is usually not ruined but will need re-recorded.

Some of the more common EMF generators are:

* Large electric transformers such as a power company worker may encounter.
* Large electric motors used in heavy industry.
* Electric welders found in heavy industry.
* MRI (Magnetic Resonance Imaging) unit found in hospitals.
* CT scanning units (CAT Scan) in hospitals.
* Even laying your ATM card on top of the TV, near the screen, could erase it.

Hope this helps!