A noun used directly after another noun to mean the same thing. An appositive is considered non-essential and is set off with commas unless it is a single word closely connected to the preceding noun. See Example 2. My neighbor, an energy conservationist, tries very hard to eliminate humidity and humidity-related problems from his home. My wife Marilyn gets concerned about humidity when fog or frost forms on the windows of the house.
Explanatory expressions add information to the sentence.
You must determine if these expressions are necessary to the clarity of the sentence. If not, they are non-essential and should be set off with commas. Add all necessary commas. During cold months like January central heating may be used in the home. My sister Sylvia finds that her heating system dries the air in her home.