(Reuters) - United Airlines Holdings Inc UAL.O said on Friday it would introduce eight new routes and add more flights to 19 destinations in the Caribbean, Mexico and Latin America. The U.S. carrier said it would begin selling tickets for new flights from Denver, Los Angeles, San Francisco, Washington D.C. and Cleveland from Oct. 17. The risk of exposure to the novel coronavirus on flights is very low, a U.S. Department of Defense study found on Thursday, a positive sign for the airline industry as it tries to rebound from the pandemic’s crushing effect on travel. Airline executives have signaled a slow but steady improvement in leisure demand but do not foresee a recovery to 2019 levels for at least two years.