Shelter · Shelter-in-Place
During an extended outage, the risks inside your home shift from obvious ones like food and temperature to quiet ones you cannot see: sewer gas, mold, and depleted oxygen.
The three risksWhy this matters
Under normal conditions, your HVAC system circulates and filters indoor air. Exhaust fans remove moisture from bathrooms and kitchens. Running water keeps drain traps sealed. None of these systems work during an extended power outage, and the consequences build gradually enough that most people do not notice until the symptoms arrive: headaches, irritated eyes, a persistent smell, or a cough that was not there on day one.
Three distinct air quality problems develop during an extended outage. Each has a different cause, a different timeline, and a different fix. Understanding all three is what keeps a shelter-in-place situation from slowly becoming unhealthy.
Risk 1
Every drain in your home, including sinks, tubs, showers, and floor drains, has a U-shaped pipe underneath called a P-trap. The water sitting in that curve creates a seal that blocks sewer gas from entering your living space. Under normal use, every time you run water, the trap refills automatically. During an extended outage, drains you are not using stop receiving water, and the trap water evaporates.
A typical P-trap can dry out in two to three weeks without water flow, faster in dry winter air or heated spaces. Once the seal breaks, hydrogen sulfide (the "rotten egg" smell) and other sewer gases, including methane and ammonia, flow freely into the home. Even at low concentrations, hydrogen sulfide causes headaches, eye irritation, and respiratory discomfort. At higher concentrations it can impair your sense of smell, meaning you stop detecting the gas even as exposure continues.[1]
How to prevent it
Pour mineral oil into every unused drain. A few tablespoons of mineral oil floats on top of the trap water and forms a thin film that prevents evaporation. It will not damage pipes, it will not go rancid (unlike vegetable or cooking oils), and it washes away harmlessly when water service resumes. This is a standard technique used in vacation homes and seasonal properties left unoccupied for months.
Cover unused toilets. Press a sheet of plastic wrap over the bowl, then lower the seat to hold it in place. This creates an air seal without using water or oil. For toilets you are actively using with bucket flushing, the trap refills each time you flush.
Do not overlook floor drains. Basement and laundry room floor drains are the most commonly forgotten traps. They dry out first because they receive water least often.
Risk 2
Your HVAC system does more than heat and cool. It removes moisture from indoor air. Without it, humidity rises. The EPA recommends keeping indoor humidity between 30 and 50 percent. Above 60 percent, mold growth accelerates significantly on any organic surface that also has dust or debris, which in a residential home means nearly every wall, ceiling, and floor surface.[2]
The CDC and EPA advise that mold can begin growing on wet or damp materials within 24 to 48 hours.[3] During an extended outage, two conditions create the moisture mold needs. In humid summer weather, the absence of air conditioning allows indoor humidity to climb unchecked. In cold winter weather, exhaled breath and body moisture condense on cold walls, windows, and ceilings, especially in the single room where the household has concentrated.
Keeping humidity manageable
Ventilate. Open windows on opposite sides of the home or room to create cross-ventilation. Even a one-inch gap on two sides moves meaningful air volume. In winter, balance ventilation against heat loss: a brief airing (10 to 15 minutes, two to three times daily) reduces moisture without collapsing indoor temperature.
Wipe condensation. Cold windows, metal surfaces, and exterior walls collect moisture from warm indoor air. Wipe them dry at least once daily. Moisture left standing for more than 24 to 48 hours on any surface is an invitation for mold.
Limit moisture generation. Cook with lids on pots. Hang wet laundry outside or in a ventilated area, not in the occupied room. Avoid drying clothes over a heat source inside a sealed room.
Monitor if you can. A battery-powered hygrometer (under $20 at hardware stores) gives you an actual number to manage toward. Below 50 percent is the target. Above 60 percent is the action threshold.
Risk 3
Carbon monoxide gets the attention it deserves: it is the leading cause of poisoning deaths during power outages, and our Generator Safety and Heating During Outages guides cover the placement and ventilation rules in detail. But CO is not the only gas that matters inside a sealed room.
Every fuel-burning device, including heaters marketed as "low-emission" and those equipped with oxygen-depletion sensors (ODS), consumes oxygen and produces carbon dioxide as part of normal combustion. In a room with good airflow, this is not a problem. In a sealed room during a winter outage, CO2 builds while oxygen decreases. The CPSC warns that an oversized heater in an insufficiently ventilated space can deplete available oxygen, and that proper ventilation is required for all combustion heaters.[4]
CO2 buildup produces symptoms before oxygen depletion itself becomes dangerous: headaches, drowsiness, difficulty concentrating, and impaired judgment. These are easy to mistake for fatigue or hunger during a stressful outage. The National Research Council of Canada notes that high CO2 concentration can cause unconsciousness or death before the corresponding oxygen deficiency would have serious effects on its own.[5]
The ventilation rule for all combustion heating
If it burns fuel, it needs fresh air. Crack a window at least one inch on the same floor as the heater. Keep interior doors open to the rest of the house to allow air circulation. Install and maintain a working CO alarm on every level. These rules apply to propane heaters, kerosene heaters, wood stoves, fireplaces, and any other device that produces flame or combustion, regardless of what the packaging says about emissions.
Managing the tradeoff
The instinct during a winter outage is to seal the house tight against cold. That instinct works against you when indoor air quality is degrading. The solution is not leaving windows wide open all day. It is brief, intentional ventilation at planned intervals.
Open windows on opposite sides of the occupied room for 10 to 15 minutes, two to three times daily. This flushes stale air and excess moisture without losing all accumulated heat. Close windows and reheat afterward. The heat loss from a brief airing is far smaller than the health cost of breathing degraded air for days.
Open windows as much as practical. Without air conditioning, your primary tools for temperature management are shade, cross-ventilation, and evaporative cooling (a wet cloth hung over an open window in dry heat). In humid conditions, airflow is even more important for preventing mold, even though it will not cool the space the way it does in dry climates.
Maintain a minimum one-inch window gap on the same floor as the heater at all times while it is running. This is non-negotiable. The window gap provides combustion air and prevents CO2 accumulation. Keep interior doors open so air circulates beyond the immediate room. When the heater is off, you can close the window to retain heat.
Related guides
When the Power Stays Off
The full timeline of what changes during a two-week outage.
Heating During Outages
Safe heat sources, CO safety, and the oxygen depletion warning.
Sanitation Without Water
Drain trap protection, bucket flushing, and sewer backup prevention.
Smoke Alarms and CO Detectors
Placement, types, and the maintenance that keeps them working.
Sources