By Chad Wiswall, Owner & Lead HVAC Technician, Alabama HVAC License #92244
If you spend any time in Central Alabama in March, April, or May, you know that yellow film on every car and porch is doing the same thing to the inside of your house. Pollen is one of three main indoor air quality challenges we deal with in this climate (humidity and dust mites are the other two), and your HVAC system is either part of the solution or quietly making the problem worse. This guide walks through what is actually in your indoor air, how your HVAC moves it around, and what equipment and habits actually fix it. This guide is part of our complete guide to HVAC in Central Alabama.
What is actually in your indoor air
The Environmental Protection Agency has consistently found that indoor air can be 2 to 5 times more polluted than outdoor air. In Alabama, the dominant indoor air contaminants in a typical home are:
Pollen. Tree pollen (oak, pine, cedar, birch) dominates February through April, peaking in March. Grass pollen takes over May through July. Ragweed and other weed pollens run August through October. Pollen particles range from 10 to 100 microns in diameter, which is large enough that a decent filter can catch them.
Mold spores. Always present in Alabama outdoor air at significant concentrations because of our humidity. Spores range from 1 to 30 microns. Indoor mold growth becomes a real problem above 60 percent relative humidity (see our humidity control guide).
Dust mites. Microscopic arachnids that thrive in 70+ percent RH and live in mattresses, carpet, and upholstery. Their droppings and body fragments are the actual allergen, and they are smaller than 10 microns.
Pet dander. Skin flakes shed by dogs and cats. Most particles are 5 to 10 microns, but the truly problematic protein fragments can be smaller than 2.5 microns.
Combustion byproducts. Gas stoves, gas water heaters, gas fireplaces, and tobacco smoke all produce ultrafine particles (below 1 micron) along with nitrogen dioxide and carbon monoxide.
Volatile organic compounds (VOCs). Off-gassing from paint, new carpet, particleboard furniture, cleaning products, air fresheners. Gases, not particles, so filters do not catch them.
The HVAC system is the single largest mover of indoor air in your house. Every time the system runs, it circulates the entire indoor air volume through the return ductwork, across the filter, and back into the conditioned space. That makes the filter, the system design, and any additional air quality equipment critically important.
How to read a MERV rating (and what your system can actually handle)
Filter performance is rated on a MERV scale (Minimum Efficiency Reporting Value) from 1 to 16. Higher MERV means smaller particles captured, but also higher airflow resistance.
- MERV 1 to 4: Catches large particles (carpet fibers, large pollen). Basic fiberglass filters from the hardware store. Almost worthless for allergen control.
- MERV 5 to 8: Catches pollen, mold spores, dust mite debris, pet dander. The standard pleated filter most homes should be running.
- MERV 9 to 12: Catches fine dust, lead dust, smaller particles. Good for allergy/asthma households.
- MERV 13 to 16: Catches bacteria, smoke, smaller ultrafine particles. The standard now required in many hospitals and commercial buildings.
Here is the practical problem: most residential HVAC systems built before 2015 were designed for MERV 8 to 11 filters. Dropping a MERV 13 into a system that was not designed for it can restrict airflow enough to:
- Cause the evaporator coil to freeze in cooling mode
- Trip the high-limit safety on a gas furnace
- Reduce capacity 10 to 20 percent
- Eventually burn out the blower motor
I see this every spring. A homeowner reads online that MERV 13 is "the COVID filter" and slaps one into a 2008 system with a 1-inch filter slot. Two weeks later they call me because the AC stopped cooling. The fix is usually to step back down to a high-quality MERV 8 or 10 pleated, OR to install a thicker media cabinet (4 to 5 inch) that can run higher MERV without restricting airflow.
For most Central Alabama homes I work on, the right answer is:
- 1-inch filter slot: MERV 8 to 11 pleated, replaced monthly in spring and summer
- 4 or 5-inch media cabinet: MERV 13 to 16, replaced every 6 to 12 months
- Allergy/asthma household with proper sizing: MERV 13 in a 4-inch cabinet, plus consider an HEPA bypass unit or whole-house electronic air cleaner
The Alabama pollen season alone justifies upgrading to a thicker media cabinet on most homes. We install Aprilaire, Honeywell, and Trane media cabinets routinely.
Humidity is half the air quality story
We covered humidity in detail in our humidity control guide, but the IAQ angle deserves its own callout. The EPA's guidance is to keep indoor relative humidity between 30 and 50 percent year-round. Below 30 percent (winter problem, not summer) you get dry skin, static electricity, and respiratory irritation. Above 50 percent, the problems compound:
- Dust mite populations explode above 55 percent RH
- Mold colonies establish above 60 percent RH
- Bacteria and viruses survive longer in humid air
- The air just feels worse, prompting lower thermostat setpoints
In Alabama summer, the practical IAQ ceiling is 50 to 55 percent RH. Getting there requires either a properly sized variable-speed HVAC system with extended runtimes, OR a dedicated whole-house dehumidifier.
Pollen-specific strategies for Alabama spring
March, April, and May are when our phones ring for IAQ complaints. The standard Alabama pollen-season protocol I tell every customer:
Change the filter every 3 to 4 weeks during peak pollen. Pollen loads filters fast. A filter that looks "still OK" at 8 weeks in February is choked at 4 weeks in April.
Keep windows closed and run the AC even on mild days. I know spring weather in Alabama tempts you to open the windows. Doing so brings the entire outdoor pollen load inside. Run the AC, even at higher setpoint, to keep the house under positive(ish) pressure and the pollen out.
Wipe down hard surfaces 2 to 3 times a week. Pollen settles on every horizontal surface. Damp microfiber cloth, not a feather duster (which just redistributes it).
Take shoes off at the door. Single biggest pollen vector into the house in spring. A doormat and a shoes-off rule cut tracked-in pollen by 60 to 80 percent.
Wash bedding weekly in hot water. Pollen, dust mite debris, and pet dander all collect in bedding. Hot water (130+ degrees) kills dust mites and removes most allergens.
Consider a portable HEPA air cleaner in the bedroom. A true HEPA portable rated for your bedroom size (most are 250 to 500 square feet) can drop allergen concentrations 40 to 70 percent in the room where you sleep. Coway, Levoit, and IQAir are reliable brands. Run it on low while you sleep.
Whole-house IAQ equipment we install
Beyond the filter and a dehumidifier, several add-on systems can substantially improve indoor air quality. None are necessary for everyone, but each has its place.
Whole-house HEPA bypass filter
A side-mounted HEPA unit that pulls a portion of return air through a true HEPA filter (99.97 percent capture at 0.3 microns) and returns it to the supply ductwork. Cleans roughly 30 percent of the airstream per pass, so over multiple cycles per hour the entire house benefits. Most appropriate for severe allergy or asthma households.
Electronic air cleaner
Uses an electrical charge to attract particles to a collection plate. Captures down to about 0.01 microns in theory, though performance drops as the plates load with dirt. Requires monthly plate washing. Trane CleanEffects and similar units are common installs.
UV-C light at the coil
A UV-C lamp installed at the evaporator coil sterilizes mold, mildew, and bacteria that try to colonize the coil surface (a real problem in Alabama humidity). Does not clean the air directly, but prevents the coil from becoming a microbial reservoir that the blower then circulates through the house. Most cost-effective IAQ add-on for many homes, especially older systems where coil bio-growth is already evident.
Ventilating dehumidifier
A relatively new category. Combines a whole-house dehumidifier with a fresh-air intake that brings in filtered outdoor air at controlled volumes. Helps with VOC dilution and CO2 buildup in tight, modern homes. Overkill for most older Alabama homes, but the right answer for new construction with very tight building envelopes.
Bipolar ionization (Needlepoint NPBI)
Released positive and negative ions into the airstream that supposedly cluster around airborne contaminants and drop them out of the air. The technology has had mixed independent test results, and the EPA has been cautious in their endorsement. I install them when customers specifically request them but I do not push them aggressively.
What we typically recommend
The most common IAQ package I install for a Central Alabama home with moderate allergy concerns:
- Upgrade to a 4-inch media cabinet with MERV 11 to 13 filter
- Add a UV-C lamp at the evaporator coil
- Verify and seal duct leaks (huge source of attic and crawlspace contaminants entering the airstream)
- Add a whole-house dehumidifier if RH is consistently above 55 percent
Typical installed cost for this package on an existing system runs $1,800 to $3,400 depending on layout and brand. Substantially less than replacing the entire HVAC system, with most of the IAQ benefit.
For severe allergy/asthma households or homes with chemical sensitivities, we layer in HEPA bypass and ventilating dehumidifier components on top of the standard package.
FAQ
What MERV rating should I use in my home?
Most residential HVAC systems with a 1-inch filter slot are designed for MERV 8 to 11. Going higher than MERV 11 in a 1-inch slot often restricts airflow enough to damage the system. If you need MERV 13+ for allergy or asthma reasons, install a 4-inch or 5-inch media cabinet first. Talk to a licensed contractor before upgrading filter MERV beyond manufacturer specifications.
Does my air conditioner filter the air?
The AC filter primarily protects the equipment, not your lungs. A standard MERV 8 pleated filter catches roughly 70 to 85 percent of particles 3 microns and larger, which covers most pollen, mold spores, and pet dander. For finer particles (smoke, ultrafine, bacteria) you need MERV 13+ filtration in a properly designed media cabinet, or supplemental equipment like a HEPA bypass unit.
How often should I change my filter during Alabama pollen season?
Every 3 to 4 weeks during peak pollen (February through May). Pollen loads filters faster than dust does. A 1-inch pleated filter that lasts 60 days in winter often only lasts 25 to 30 days during heavy pollen season.
Do air purifiers actually work for Alabama allergies?
A true HEPA portable air purifier sized correctly for the room can drop allergen concentrations 40 to 70 percent in that room. They are most effective in bedrooms, where you spend 7 to 9 hours per night. Whole-house solutions like media cabinet upgrades and HEPA bypass units affect the entire home's air more uniformly.
Should I install a UV light in my HVAC system?
UV-C lights at the evaporator coil are one of the most cost-effective IAQ add-ons for Alabama homes because our humid climate makes coil-surface mold and mildew growth a real problem. Installed cost typically runs $400 to $700. The lamp does not clean room air directly, but it prevents the coil from becoming a contamination source.
Worried about allergies, mold, dust, or general air quality in your Central Alabama home? Chad's AC Direct installs and services full IAQ packages from filtration upgrades to whole-house dehumidifiers, UV-C systems, and HEPA bypass units. Two locations serving 16 Central Alabama cities since 1993: Montgomery (2546 Bell Rd, 334-264-6464) and Dadeville (360 Windflower Dr, 334-478-1438). Alabama HVAC Contractor License #92244, BBB Accredited A+, 1,247 reviews at 4.9 stars. Buy Direct, Pay Less.