5 years old my son nose bleeding, what's reason...

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CAUSES OF NOSE BLEEDING:
- trauma
- nose picking
- dry air which leads to irritation and dehydration in the membranes of the nose
- allergies
- sinus infection
- cold
- bacterial infection
- blowing the nose too hard
first aid:
- Keep the kid in an upright position
- ask the kid to concentrate to breath from his mouth
- pinch the nose for 10-15 minutes straight for the hemostasis of the vessels
- apply ice pack on the nose
treatment:
- the treatment depend on the cause
- stop the kid from the habit of always picking his nose 
- make sure there is no broken bones or pain or constant bleeding
- decongestant in case of allergies or infections
- you should consult the doctor if the causes was related to any kind of infections or the appearances of any related symtoms 
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Kind Regards