U.S. exchange-traded fund assets totaled $1.2 trillion as of August 31 - up 16.6% year-to-date, according to State Street Global Advisors' ETF Snapshot.

In August, investors added $4.7 billion more to ETFs than they withdrew, as fixed income and commodity ETFs attracted $4 billion and $2.4 billion of inflows respectively, according to SSgA.

Large cap ETFs experienced the most significant outflows with $4.4 billion leaving the category during the month. Year-to-date, ETFs have attracted over $93.4 billion of inflows.