After a calamitous month of roller-coaster ebbs and flows, the U.S. stock market suffered its third-worst loss of the year Tuesday, prompted by a big selloff in stocks in Asia and Europe. The Dow Jones Industrial Average fell almost 500 points while the Nasdaq fell 140 points. Some speculate that the recent Tianjin explosion could have led to economic instability in China. The U.S. futures market was up in Asia trading as of 4 p.m. local time in Tokyo.