Asian stocks took a big hit and the same setup could hurt US investors next.

• Over $600 billion was wiped off Asian markets, with Japan down more than 4% and Taiwan, Hong Kong, and China also falling.
• South Korea's market was spared a deeper drop only because it was closed for a holiday.
• The bigger worry is US margin debt — money investors borrow to buy stocks — which just hit its highest share of the economy on record.
• Every past spike in borrowed money to buy stocks was followed almost immediately by a crash.
• The real amount of borrowing is likely even higher than the official number shows.

Outlook: With borrowed money in stocks at record highs, US markets look fragile and a sharp drop is the main risk ahead.