International Monetary Fund Reuters As central banks around the world have loosened monetary policy over the past decade, corporations have felt encouraged to pursue "financial risk-taking," according to a report from the IMF published in October. That dynamic has made some systemically important economies more vulnerable to an economic slowdown, the report found. "Corporate leverage can also amplify shocks, as corporate deleveraging could lead to depressed investment and higher unemployment, and corporate defaults could trigger losses and curb lending by banks," the IMF wrote. In a recession half as severe as the 2008 financial crisis, corporate debt-at-risk - which is owned by firms that can't cover interest expenses with their profits - could increase to $19 trillion, or almost 40% of total corporate debt in major economies, the IMF said.

BlackRock Thomson Reuters The trillion-dollar asset manager said in an October report that BBB-rated bonds, the lowest bracket of the investment-grade debt, accounted for more than 50% of the market, compared with 17% in 2001. As demand for investment-grade debt has grown, the creditworthiness of issuers has fallen. According to BlackRock, leverage levels are creeping toward the highest readings since 1992. The firm measured leverage using a ratio of debt minus cash and cash equivalents to 12-month EBITDA. "We believe the sharp increase in the proportion of BBB-rated constituents has made the investment-grade bond sector riskier than in recent years," BlackRock wrote. "BBB-rated bonds are typically the most vulnerable of all investment-grade debt in a recession." If BBB-rated bonds were downgraded, they would then be considered high-yield debt, or "fallen angels," which could cause their value to fall, the firm added.