I leave the each rebounder in the hot sun and see which one smells like rubber, formaldehyde, and petrochemicals. I have also taken a hair dryer to see if any part of the rebounder off gas. Polymer feet do not off gas like rubber. I also wanted to see if a naugahyde cover off gassed like vinyl. I also found out if polyester threading off gassed like nylon threading. For those that cannot tolerate going into a tire store and smelling all the rubber, please make sure you know which rebounders don’t have this smell before you buy. If you have any sensitive to rubber, vinyl, or nylon, please let me know. Some new rebounders just came out of the factory and may have a strong smell while others have been sitting for awhile and have off gassed before it is delivered to the customer. Note: I do not have a problem with companies that use nylon mat covers or nylon threading as long as it has had a chance for 100% of the formaldehyde gas or any other smells to off gas. For those that get a rebounder and do not want a cover, I can take off some of the price and have the manufacturer take off the cover. Those that are buying single purpose rebounders usually don’t need the cover since they aren’t jumping and losing balance.