Allergies or Mold? How to Tell the Difference This Spring

It usually starts the same way. You wake up congested, your eyes itch by lunch, and by dinner, you’re convinced it’s the worst spring allergy season of your life. But there’s a question worth asking before you reach for another antihistamine: Is it actually allergies — or is it mold growing somewhere in your home?

The symptoms overlap almost completely. The cause, the treatment, and the cost of ignoring it do not. Here’s how to tell the difference, what to watch for in Chicagoland and St. Louis specifically this May, and when it’s time to stop guessing and have your home tested.

Allergies vs. Mold: The Side-by-Side

The fastest way to narrow it down is to look at when and where your symptoms hit — not just what they feel like.

If you read that table and your symptoms cluster in the right column — especially “worse indoors” and “improves when you leave” — that’s the single most reliable signal that something in your home is the problem, not the pollen outside.

The Symptom Decoder

Some specifics worth paying closer attention to:

Lean toward allergies if: Your symptoms started the week of a heavy tree-pollen bloom, suddenly. They’re worst before 10 a.m. or on windy days. Cetirizine or loratadine clears you up. You feel notably better in air-conditioned indoor spaces with the windows closed.

Lean toward mold if you’ve had “allergies” for months rather than weeks. You wake up congested but feel better after a few hours at work. You notice a musty smell in one specific room. Has there been any history of a leak, slow drip, ice dam, basement seepage, sump pump failure, or roof issue — even one you thought was fixed? Antihistamines help only partially. You have a persistent cough or wheeze that doesn’t fit the typical hay fever profile.

Strong mold red flags: Visible discoloration on drywall, baseboards, grout, or HVAC vents. Warped flooring. Bubbled paint. A “wet cardboard” or “old basement” smell. Condensation on windows that doesn’t resolve. Worse symptoms after running the A/C or furnace fan.

What’s Actually in the Air Right Now

May is one of the worst months to try to self-diagnose because the outdoor and indoor triggers stack on top of each other. Here’s what’s happening locally.

Chicago: Tree + Grass Overlap and a Damp Summer Setup

Tree pollen runs from February through May in Chicagoland, and grass pollen kicks in starting in May and runs through July. This year, Loyola Medicine’s allergy experts have flagged 2026 as a season where tree and grass pollen are arriving in overlapping waves rather than handing off cleanly — meaning May symptoms can be more intense than usual.

At the same time, the risk of indoor mold is rising. Chicago summer humidity routinely sits at 70% or higher, and indoor relative humidity above 60% is enough for mold colonies to take hold on drywall, framing, and HVAC components. The EPA and CDC recommend keeping indoor humidity between 30–50%. If your home doesn’t have a dehumidifier or a working HVAC condensate system, May is when the gap shows up — and so do the symptoms.

The Chicago tell: If you feel worse after rain rather than during it, mold is the more likely culprit. Pollen counts drop during rain events; mold spore counts spike in the hours and days after.

St. Louis: Oak Pollen Winds Down, Mold Spores Wind Up

St. Louis tree pollen — oak especially — peaks earlier and tapers through May. Grass pollen takes over from mid-May through July. But the bigger spring story is mold: the St. Louis County Pollen and Mold Center has tracked May and early-June outdoor mold spore counts in the 5,000–20,000 range — high enough to cause symptoms in some people without anything wrong inside the home.

What makes St. Louis distinctive is the indoor amplifier. Spring rains, basement seepage, and Mississippi/Missouri river-system flooding push moisture into homes that may have looked fine three weeks earlier. Mold can establish a colony in 24 to 48 hours once a surface stays damp. Add summer humidity hovering between 63–75%, and a small spring water event becomes a months-long indoor air problem.

The St. Louis tell: Walk into your basement first thing in the morning. If the air feels noticeably heavier or smells different than the upstairs, your home is holding moisture — and your sinuses are probably reacting to it.

When to Test Your Home (And When You Probably Don’t Need To)

Most people don’t need a mold inspection. Some do. The line is usually clearer than people think.

You probably don’t need testing if: Your symptoms are clearly seasonal, follow the pollen forecast, fully resolve with allergy medication, and you’ve had no leaks, no flooding, no humidity problems, and no visible signs of moisture in the home.

You should consider an inspection if any two of these are true:

• Symptoms persist for more than three weeks despite allergy treatment
• Symptoms are noticeably worse indoors than outdoors
• There’s been a water event in the past 12 months — leak, flood, ice dam, appliance failure, sump pump problem
• You see, smell, or suspect mold anywhere in the home
• A household member has asthma, immune issues, or unexplained respiratory symptoms
• Your basement, crawlspace, or attic has visible moisture or a musty smell
• Symptoms began or worsened after moving in, renovating, or starting up the HVAC for the season

A professional inspection is not the same as a hardware-store DIY mold test. Petri-dish kits will almost always show some mold (it’s everywhere in trace amounts) and tell you very little about whether you have a real problem. A certified inspection looks at moisture sources, maps where mold is likely growing, and uses air and surface sampling only where it actually answers a question.

The Short Version

If your spring symptoms get better when you leave the house, improve outside, and respond cleanly to over-the-counter allergy meds — you’re probably looking at pollen. Manage the season, and you’re fine.

If your symptoms hang on, get worse indoors, ignore your medication, or come with a smell, a leak history, or a basement that just “feels off” — you’re not dealing with allergies. You’re dealing with what’s in the building, and it’s going to keep making you sick until someone inspects it.

That’s the part we can help with.

Get an Inspection in Your Market

You don’t need to know whether it’s mold before you call. That’s what the inspection is for.

Sources: Loyola Medicine Allergy Count, St. Louis County Pollen and Mold Center, U.S. EPA, CDC, Mayo Clinic, American Academy of Allergy, Asthma & Immunology.