For avoiding poisoning effects during anesthesia, the quantitative amount of an anesthetic agent degradation product, preferably carbon monoxide CO and/or trifluoromethane CHF3, in an anesthetic gas mixture is determined. When the determined quantitative amount of the anesthetic agent degradation product in the anesthetic gas mixture exceeds a given threshold, an alarm is provided. This is preferably accomplished by measuring a Raman spectrum of the gas mixture, and determining the quantitative amount of the anesthetic agent degradation product in the gas mixture by comparing the measured Raman spectrum with a reference spectrum of the anesthetic agent degradation product.