According to a recent study:

• 6.8% of emergency department visits were followed by returns to the emergency department within 7 days
• 5.7% of these returns were adverse events
• 56.6% of the returns were preventable

Source: "Adverse events in patients with return emergency department visits," BMJ Quality & Safety, December 24, 2014, http://qualitysafety.bmj.com/content/early/2014/12/24/bmjqs-2014-003194.short?g=w_qs_ahead_tab