Sustainable ETFs are on the rise and could move impact investing into the mainstream, say experts. : Exchange-traded funds (ETFs) that invest in securities with sustainability goals are growing in popularity. According to predictions from BlackRock, the world’s largest asset management firm, sustainable ETF fund assets are poised to grow from the current $25 billion to more than $400 billion by 2028. Millennials, in particular, are attracted to these ETFs because they offer low fees and broad diversification while catering to social impact goals. Sustainable or impact investing are normally the realm of high net worth individuals.