We have been using the 36 watt Swordfish UVC on our furnace for the past 7 years since 2009. During this time nobody in the house has got the flu or more than a mild cold.
I probably pays for itself just in the saved tissues you use. And not having a sore throat all winter is great too. We do not have allergies but it as also highly effective for that. For best results the bulb should be replaced every year or so. It has a reminder light built in.
You can buy this direct from the company or through Canadian Tire and other dealers.
Their customer service is also excellent.
Note to Homestars - the proper category for this would be air purification if you had that category - you have water purification