A University of Iowa study concludes smokers wind up paying more than non-smokers when it comes to having surgery in a hospital. The study found the average hospital bill for a smoker who undergoes general surgery averages about $900 more than the hospital bill for someone who has never smoked.

That’s a 4% difference in the bottom line for smokers and non-smokers. According to University of Iowa researchers, about 30% of Americans who undergo surgery are smokers. The U-of-I study also found ex-smokers fared better and the cost of surgeries for former smokers and people who’ve never smoked was roughly equal.

The study is published in the June issue of “Journal of American College of Surgeons.” Nearly 15,000 patient records were examined in the research.