Hmm, I wonder how effective that would be. My small home's ventilation system is currently taking in about 100m³ of fresh air per hour. That's about 1600L/minute. Let's assume the intake duct is about 20cm in diameter, and assume we can place a bulb to illuminate a 50cm stretch of pipe. That's a volume of about 16 liters. So the air would be in visible range of the bulb for about half a second. Would that be enough for a UV bulb to do anything?

This was back of napkin maths, could be way off.

Edit: sanitizing incoming air is probably pretty useless, as the outdoor concentration of pathogens is likely very low to begin with.