Not just rank but also damp. All lofting insulation loses some effectiveness when wet so it is important to air out and dry out you bag if you are using it over a long period of time or if it gets wet. You can also see people on the trail doing the same thing during a sunny break if it has been raining for while or it is a damp climate.
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If I wouldn't eat it at home, why would I want to eat it on the trail?