Puerto Rico is drowning in $72 billion of debt it admittedly cannot pay back. Several states — Illinois, Pennsylvania, New Jersey and Kentucky among them — are facing mounting financial problems of their own, mainly because of pension promises that are not properly funded.

Those government travails come just two years after Detroit’s historic bankruptcy, the largest municipal default ever.

Since individuals hold most of the $3.7 trillion invested in municipal bonds — or about 70 percent, either directly or through mutual funds — it raises the question: Should investors be worried? After all, municipal bonds have traditionally been viewed as safe investments.

“There is more stress in the muni market today than there was 10 years ago because there are higher fixed costs like pensions and retiree health care costs, increased debt costs and more modest revenue increases,” said Lisa Washburn, a managing director for Municipal Market Analytics, a research firm based in Concord, Mass. “I am more worried about credit deterioration in states with significant pension issues, but I am not at this point concerned about any risk of default at the state level.”