Both Charcoal and Propane emit carbon monoxide but Propane is much cleaner burning than Charcoal so that makes Propane a much better option for grilling with to keep the air clean and healthy.
But I love cooking with Charcoal Better because it gives the food a much better taste than propane but if you just want to burn a cleaner fuel for cooking food on the grill then Propane would be the cleaner option for the environment.