The way that these ETFs work is that the managers of the funds issue creation baskets in increments of a certain number of units and offer them for sale to authorized purchasers, which are generally large brokerage firms.  However, sometimes the fund can hit a limit of units and “freeze” trading, in the case of the natural gas ETF.

This regulation is being imposed at a time when the Obama administration is trying to increase transparency and regulation of the financial markets.

For more stories on commodities, visit our commodities category.

Kevin Grewal contributed to this article.