Heating season changes how a house breathes. Windows stay shut, the house tightens up, and whatever the stove puts into the air stays around longer. In homes that cook with gas or propane, that includes nitrogen dioxide (NO2), along with carbon monoxide and fine particles. The range hood is the main defense, and it's often the most misunderstood piece of ventilation in the house.
What the research shows about gas stoves and NO2
EPA lists unvented combustion appliances like gas stoves as a primary indoor NO2 source. It notes that in homes with gas stoves, indoor levels "often exceed outdoor levels," and that low-level exposure is linked to increased bronchial reactivity in some people with asthma and more respiratory infections in young children (EPA). EPA's most effective fixes are venting to the outdoors and making sure combustion appliances are correctly installed and maintained (EPA).
A 2024 Stanford study that measured more than 100 homes estimated that typical gas or propane stove use raises average yearly NO2 exposure by about 4 parts per billion. Concentrations spiked during and after cooking even with a vented range hood running, sometimes reaching bedrooms within an hour (Stanford University). People in homes under 800 sq ft had about twice the average exposure (Stanford University).
Capture efficiency: the number that matters
A hood's CFM rating tells you how much air it moves, not how much of the cooking plume it catches. Researchers at Lawrence Berkeley National Laboratory measure capture efficiency: the share of pollutants sent directly outside. In lab tests of three common hood designs at high speed, two designs captured more than 90% of back-burner exhaust, but front-burner capture fell to 73% to 78%, and as low as 46% to 63% for one design (Lawrence Berkeley National Laboratory). Capture was "substantially lower" on the lowest fan setting (Lawrence Berkeley National Laboratory). LBNL's field survey also found nearly all microwave-hood combinations cover only about half the cooktop (LBNL range hood survey).
Free fix: cook on the back burners and turn the hood on before you light the burner. Based on the LBNL results, moving pots to the back and running a higher fan speed are the two easiest ways to raise capture without buying anything (Lawrence Berkeley National Laboratory). Keep the hood running for a few minutes after you finish cooking.
What the codes and standards say
- Exhaust rate: ASHRAE Standard 62.2 calls for at least 100 cfm for a vented range hood that's switched on as needed. Other kitchen exhaust fans, including downdraft, need 300 cfm or 5 air changes per hour in an enclosed kitchen (ASHRAE Standard 62.2).
- Makeup air: Under the model residential code, if a home has a fuel-burning appliance inside the air barrier that is neither direct-vent nor mechanically vented, any exhaust system capable of more than 400 cfm must have makeup air at roughly the same rate, through a damper that opens automatically (IRC M1503.6 (2021 text)). Local codes and editions vary.
The reason for that makeup air rule: a powerful hood in a tight house can pull air back down a chimney, including from an older natural-draft water heater or furnace. Bigger isn't automatically better.
Kitchen ventilation checklist
| Check | Why it matters |
|---|---|
| Hood ducted outdoors, not recirculating | Recirculating hoods filter grease, not combustion gases. EPA's top fix is venting outside (EPA). |
| Hood covers the front burners | Front-burner capture is the weak spot (Lawrence Berkeley National Laboratory; LBNL range hood survey). |
| At least 100 cfm delivered, not just rated | The ASHRAE minimum for vented hoods (ASHRAE Standard 62.2). Long, kinked or undersized ducts lower real airflow. |
| Makeup air for hoods over 400 cfm | Required where natural-draft appliances are present (IRC M1503.6 (2021 text)). Prevents backdrafting. |
| Natural-draft water heater or furnace nearby | Have a combustion safety and draft test done after any hood upgrade. |
| Working CO alarm | A basic safety layer in any home with combustion appliances. |
Limitations
Lab capture results depend on hood design, mounting height and fan speed, so your hood may perform better or worse. The Stanford estimates are modeled national averages. Your home's levels depend on its size, layout, ventilation and how you cook. Electric and induction cooktops don't produce combustion NO2, but cooking itself still produces particles that call for good ventilation.
This article names no products and endorses no manufacturer. HVAC Zone Inc is a multi-brand dealer, and the Hub is brand-neutral.
Upgrading a range hood or worried about backdrafting from your water heater or furnace? Request a consultation and we'll check your kitchen exhaust, makeup air and combustion safety, across all major brands.