NEW YORK (PNN) - July 25, 2015 - Emerging-market currencies are in free fall.

An index of the major developing-nation currencies fell to an all-time low this week, extending its drop over the past year to 19%, according to data compiled by Bloomberg going back to 1999. The Russian ruble, Colombia's peso, and the Brazilian real have fallen more than 30% over the past year for some of the worst global selloffs.

China's economic slowdown is pushing down commodity prices, weighing on raw-material exporters from Brazil to Mexico and South Africa. Adding to the pain is the expectation that the Federal Reserve will soon embark on its first interest rate increase since 2006, threatening to lure capital away from developing nations.

“This combination of a soft landing in China and a Fed that will normalize rates soon poses significant risks to emerging markets, especially their currencies,” Stephen Jen, a former International Monetary Fund economist who is now managing partner at SLJ Macro Partners in London, wrote in a July 23 note. Jen said he expects “a violent sell-off in some emerging-market currencies in the second half this year.”

While currency depreciation tends to spur growth by making exports cheaper, so far this is not happening because global trade has stalled, according to Citigroup, Inc. and UBS Group AG. The International Monetary Fund forecasts emerging markets will grow 4.2% this year, the slowest since 2009.