Sanitizing
Sanitizing has a narrow legitimate use and a wide career as an upsell, and telling them apart comes down to one question.
Short answer
A sanitizing or antimicrobial treatment applied inside ductwork has a purpose after confirmed microbial contamination has been physically removed. Applied instead of cleaning, or on a clean system as routine, it does nothing useful. The product must also be labelled for use in HVAC systems, which is a fair thing to ask.
When it has a purpose
After confirmed microbial growth has been physically removed, as a final step.
After a sewage backup or a biohazard event affecting the ductwork.
After rodent contamination, once the nesting material and droppings are out.
After a water event where duct interiors were wetted.
In a specific medical situation where a physician or an occupational hygienist has recommended it.
And in all of those, after removal rather than instead of it.
The one question
Ask what is being treated and where it is. If the answer names a specific contamination in a specific location that has been physically removed first, the treatment makes sense. If the answer is general — bacteria, allergens, "sanitizing the system" — it is an upsell on a clean duct.
When it is not
On a clean system as routine, which is the most common way it is sold.
Instead of physical cleaning, which is the version to decline outright.
Before cleaning, which coats the contamination rather than removing it.
As an allergy treatment, which it is not.
As a deodorizing measure with the moisture source still present, because the smell returns.
And as a standing annual service, which has no basis.
What to ask about the product
Is it registered and labelled for application inside HVAC ductwork, because many products are not and using them that way is off-label.
What is the active ingredient, and can I see the label and the safety data sheet.
What is the re-entry period for the household.
Is anybody in the household asthmatic, pregnant, immunocompromised or sensitive — which should change the recommendation.
Are there pets or birds, because birds in particular are sensitive to airborne chemicals.
And what happens if the smell comes back, because with the source still present it will.
Common questions
Is duct sanitizing necessary?
Only after confirmed contamination has been physically removed. Not as routine.
Does it help with allergies?
No. Filtration and source control do.
What should I ask?
What specifically is being treated and where, and whether the product is labelled for HVAC use.