I’m not saying that your smartphone is unsafe. Many things can cause harm if used carelessly, but today’s smartphone technology has more power (and heat) crammed into a smaller package than ever. Today’s smartphones get hot very easily.
This means that you should not smother them in any way that could prevent them from cooling off (pillows included, but cases are not). Recently, a girl was awakened by the smell of her Samsung Galaxy S4 burning a hole in her pillow. The phone was so badly burned that it was unrecognisable.
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The theory is that the phone overheated, causing the battery to swell and start a fire (the battery was definitely swollen). Overheated lithium-ion batteries can ignite, which is why they should be shut off immediately if you sense that they are overheating.
This could have been a case of thermal runaway, which is a chain reaction of heating events.
Samsung warns in their manuals that you are not to smother your smartphones, as that poses a risk of fire and explosion.
This might happen to laptops as well, as they can become extremely hot under some circumstances. For example: Laptops may fail to go into standby mode due to poorly designed technology or operating system malfunctions. This happens so often that you should always check your laptop to ensure it is in standby mode before putting it into a case because it will generate far more heat when it is on, causing the inside of the case (which is almost certainly not ventilated) to become extremely hot.
Ideally, I would recommend shutting your laptop down completely if you are going to put it in a case, as laptops can also wake up spontaneously while in the case, causing severe overheating, and possibly a fire or battery explosion.
Video Source: Fox 4 News – Dallas Fort Worth on Youtube.