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There is a lot of myth and misinformation out there about how magnetic fields interact with card datastrips, so here is a collecion of my research to set the record straight!
First -- YES, magnetic datastrips CAN be demagnetised. But it requires a specific set of circumstances. The older the strip, the easier it is to demagnetise. Modern card strips are quite difficult to demagnetise, and chip cards are impossible (not even an MRI or X-ray will demagnetise a chip card!)
This means, if you have strip cards, there is always a risk. So what are the conditions that will demagnetise a card? What are the risk situations to avoid?
What exactly is the datastrip, and how can it be demagnetised?
A card datastrip is actually a placement of gagillions of teeny tiny iron shards, positioned in coded patterns that are readable and decoded by the magnetic field they create. (think etch-a-sketch, only far more miniaturised!!). Sustained and direct contact with a strong magnetic field will eventually shift the shards positioning, so the code no longer makes sense to the machine reader. So there are two fundamental elements that put your cards at risk: The Proximity of the Contact and Contact sustained over time. So lets go into these one by one...
1. Direct Contact
The level of risk of "proximity" of the strip to the magnet depends somewhat on the type of magnet -- it is not so much the strength of the magnetic field, as the material of the magnet itself. Strong Ferrous magnets can damage depending on the strength vs the proximity. For credit cards, it takes a 3000+ Gaus stregnth in close contact. Disposable cards, like metro tickets or hotel keys (if the strip is brown instead of black), require much less to interfere, so need to be kept farther away.
Neo dymium magnets (which are used in Bagnetique) require direct contact with the magnet itself to have an impact.
With Bagnetique direct contact is virtually impossible because the actual magnets are behind a series of barriers: first, the Bagnetique casing itself. The magnets are behind the printed cover, preventing direct contact with the magnets. Then, most of the time, your phone is also on the magnet, creating another important barrier between your cards and the magnets.
2. Sustained contact: A casual brush up near a neo-dymium type magnetic is not enough to demagnetise a credit card. But sustained contact (with any type of magnetic field) over a long period of time can still damage a card strip. The actions of pulling your cards in and out of your wallet or your bag are quick - the way you use your cards and your bag, normally, will not created the circumstances for this kind of sustained contact. And note, this is true of any type of magnetic field -- including the one your phone creates, as well as any magnetic clasps or clips you may have in your purse or wallet.
So, whether or not you have Bagnetique, if you have a datastrip card and you want to be 100% it cannot be demagnetised, you should always keep your cards in a wallet. Cards stored in a wallet are safe from all the combinations of circumstances that could demagnetise them!
Here are some articles for further reading, if you want dig into even more details!