NH Air Quality Alerts: What Ozone Means for Your Indoor Air
Ground-level ozone is an air pollutant formed when sunlight reacts with vehicle exhaust, industrial emissions, and other combustion byproducts in the atmosphere. New Hampshire issues air quality alerts when ozone along the Seacoast, in the southern tier, and at higher elevations rises to levels considered unhealthy for sensitive groups (children, older adults, and people with asthma or other respiratory conditions).
While outdoor ozone is a regional problem managed by state and federal agencies, what happens inside your NH home is something you can control. At A.J. LeBlanc Heating, we design and install indoor air quality (IAQ) solutions that protect families across southern New Hampshire from the indoor effects of poor outdoor air days.
What is ground-level ozone?
Ozone (O3) is a gas made of three oxygen atoms. In the upper atmosphere, ozone protects life by blocking ultraviolet radiation. At ground level, the same molecule is a respiratory irritant.
Ground-level ozone forms when:
- Pollutants from vehicles, power plants, and industrial sources (nitrogen oxides and volatile organic compounds) react in the atmosphere
- Strong sunlight drives the reaction
- Warm, stagnant air allows the ozone to accumulate
This is why NH ozone alerts cluster in summer, especially during heat waves and stagnant high-pressure systems.
When NH issues an air quality alert
The NH Department of Environmental Services (DES) issues air quality forecasts and alerts when ozone (or, less commonly, fine particulate matter) is expected to reach levels that affect health. Alert levels typically progress:
- Moderate: air quality is acceptable for most people, but unusually sensitive individuals may experience symptoms.
- Unhealthy for sensitive groups: children, older adults, and people with asthma or heart disease should limit prolonged outdoor exertion.
- Unhealthy: everyone may begin to experience effects; sensitive groups should avoid prolonged outdoor exertion.
- Very unhealthy or hazardous: rare in NH, but possible during severe wildfire smoke events.
Current forecasts are posted at the NH DES Air Resources Division website during the warm season.
What to do outdoors during an alert
- Limit prolonged outdoor exercise, especially in the afternoon when ozone peaks.
- Reschedule strenuous activities to early morning when ozone levels are lower.
- Sensitive individuals should stay indoors as much as practical.
- If working outside, take more frequent breaks and stay hydrated.
What to do indoors
The most important thing you can do indoors during a bad air day is keep outdoor air outdoors:
- Close windows and doors.
- If you have central AC or a heat pump, run it in cooling mode with the fan on. The system will recirculate and filter indoor air.
- Avoid activities that create additional indoor pollutants: wood stoves, fireplaces, harsh cleaning products, indoor smoking.
Whole-home indoor air quality systems
For homes with ducted heating and cooling, several add-on systems meaningfully improve indoor air quality during bad air days (and year-round):
Higher-MERV media filtration
A 4 to 5-inch media filter rated MERV 11 to MERV 13 captures far more fine particles than a standard 1-inch filter, including most of the particulate matter from wildfire smoke and outdoor pollution that drifts indoors.
HEPA filtration systems
For homes with serious respiratory concerns, a whole-home HEPA system can be installed in parallel with the duct system, capturing 99.97 percent of particles down to 0.3 microns. More expensive than media filtration but the gold standard for particulate removal.
Electronic air cleaners
An electronic air cleaner uses an electrical charge to capture particles on collector plates. These units are highly effective on smoke, dust, and pollen, and the collector plates can be washed and reused. Operating cost is low (roughly a few cents per day over the life of the unit).
UV air treatment
Duct-mounted UV air treatment systems (Sanuvox is one well-known brand) target biological contaminants: mold, bacteria, viruses, and some VOCs. UV does not capture particulates, so it complements (rather than replaces) filtration.
HRV or ERV with filtration
A Heat Recovery Ventilator or Energy Recovery Ventilator exchanges stale indoor air for fresh outdoor air while recovering most of the energy. During an air quality alert, the unit can typically be switched to a low or off setting to limit outdoor air intake.
A note on portable air cleaners
Portable air cleaners vary widely in quality, so it pays to know the difference. Undersized tabletop gadgets with tiny filters move too little air to clean a room, and ozone-generating ionizers should be avoided entirely; they add the very pollutant this article warns about. A properly sized portable HEPA air cleaner is a different story: current EPA and ASHRAE guidance recognizes these units as effective for reducing particles in a single room, including during wildfire smoke events and allergy season. Match the unit's Clean Air Delivery Rate (CADR) to the room size before buying. For ducted homes, whole-home filtration and controlled ventilation remain the better permanent solution.
Talk to us about your home's air quality
If anyone in your household has asthma, allergies, or other respiratory concerns, or you simply want cleaner indoor air during NH's warm-weather air quality alerts and the increasing reality of wildfire smoke events, contact A.J. LeBlanc Heating. Serving New Hampshire families since 1928.
Frequently Asked Questions
How do I find out if today is an NH air quality alert day?
Check the NH Department of Environmental Services Air Resources Division website during the warm season. Many weather apps also display the local Air Quality Index (AQI).
Can my AC filter out wildfire smoke?
A standard 1-inch filter does very little for smoke particles. A 4 to 5-inch MERV 13 media filter, a whole-home HEPA system, or an electronic air cleaner all substantially reduce indoor smoke particles.
What is the difference between MERV 11 and MERV 13?
MERV is the rating scale for filter effectiveness. MERV 13 captures more (and smaller) particles than MERV 11, including smoke and most bacteria. The trade-off is higher static pressure across the filter, which can stress some older HVAC systems. A licensed contractor can verify your system will tolerate a higher-MERV filter before changing it.
Does running my AC during a smoke event help?
Yes, if you have an adequate filter. The system circulates indoor air through the filter, which removes particulates over time. Make sure windows and doors stay closed and the fresh-air intake (if any) is closed or limited.
What is the best indoor air quality system for asthma?
For asthma specifically, a combination of high-MERV media filtration (MERV 13 or higher) plus an HRV or ERV for controlled fresh air typically delivers the most consistent improvement. An electronic air cleaner or HEPA system can be added for additional particulate removal.