I had a similar conversation with a tenant, complaining to me that his allergies were worse...I was like, well, this house had brand new carpeting throughout when you moved in, but most people with allergies don't chose a house that is carpet throughout, and live with 2 dogs who shed so much they will clog the air filter in a month, and told him about plug-in  air purifiers, since the furnace isn't designed to clean the air, only to heat it, and those high rated filters are bad for HVAC systems according to any HVAC guy you talk to.

But hopefully an air purifier can help too.

I have a side-hustle representing a revolutionary device that:  * Restores and purifies the body by assisting the lymphatic system and helps detoxify* Prepares and supports your muscles for pre and post workouts* Reduces inflammation, reduces swelling, faster healing* Revitalizes the body with body contouring and cellulite deduction* Enhanced sleep and mental clarity...and a lot moreThis device is connected to an app uses AI to track results and progress. 

Have you ever used an air purifier or ozone generator to get rid of smells? 

But that's a $10k+ bill and I still don't KNOW if its going to solve the problem.I could really use some help here.I've had several professionals comes to the house and a lot of them say conflicting things, so its all been very confusing.I bought an air particle counter and that didn't show anything bad.I use a MERV 14 filter for my AC system intake and have a small HEPA air purifier in the house.Not a great experience with my first property, but I'm learning a hell of a lot.Thanks

Buy an ozone machine off amazon, let it run for 24 hours with no one in the unit and see if that fixes the smell.We actually did purchase an air purifier to get rid of the smell.

He mentioned that an air purifier would help with allergies.   

My background is in ecological design, which you can find out more about in my profile, but in short it means I design and help create abundant edible landscapes that not only grow healthy food, but also enhance the ecology by purifying water, providing habitat for wildlife, and actually sequestering Carbon dioxide from the atmosphere in soil and plants.I have much gratitude to the creators of this space for creating something that is truly inspiring and brings people together.  

We finally sealed all the walls, ceilings and floors with Zessner odor killing sealer which helped a lot, purchased an air purifier, and kept the house open as much as we could. 

Also @Tim Porsche spoke of an ozone generator, also consider an Hepa filter air purifier.