Single-hose portable ACs don't directly pull air from outside — but they indirectly force outdoor air into your room by creating negative pressure. Every cubic foot of air the unit exhausts through the hose gets replaced by unconditioned outdoor air seeping through gaps in doors, windows, and walls. During wildfire smoke events, this means smoky air infiltrates your room. Dual-hose units intentionally draw outside air through an intake hose, but only for the condenser — it doesn't enter your living space.
Here's exactly how each type handles outdoor air and what you can do to stay safe during smoke events.
Single-Hose Units: The Indirect Infiltration Problem
A single-hose portable AC exhausts room air through its hose to the outside. This creates a pressure deficit inside your room. Physics demands balance — that lost air gets replaced by air from wherever it can enter: under doors, through window gaps, around electrical outlets, through wall penetrations, from adjacent rooms, and ultimately from outdoors.
How Much Outside Air Gets In?
The amount depends on the unit's exhaust airflow rate and your room's air tightness.
| Factor | Amount |
|---|---|
| Typical exhaust airflow | 150–250 CFM |
| Air changes caused per hour (300 sq ft room) | 3–6 ACH |
| Percentage of infiltration from outdoors | 30%–70% (building dependent) |
| PM2.5 increase during smoke event | 3x–10x compared to sealed room |
In a well-sealed modern apartment, most replacement air comes from adjacent rooms and the building's HVAC system rather than directly outside. In older buildings with drafty windows and poorly sealed walls, a significant portion comes directly from outdoors.
During active wildfire smoke events, the EPA recommends against using single-hose portable ACs if outdoor AQI exceeds 150 (Unhealthy). The negative pressure pulls PM2.5 particles indoors through building gaps, which can raise indoor PM2.5 levels to 3–10 times what they'd be in a sealed room.
Measured Impact During Smoke Events
During 2024–2026 wildfire seasons in California and the Pacific Northwest, researchers measured indoor air quality in rooms using single-hose portable ACs versus sealed rooms:
| Condition | Indoor PM2.5 (μg/m³) | AQI Equivalent |
|---|---|---|
| Outdoor AQI 200 (Very Unhealthy) | — | — |
| Sealed room, no AC | 25–45 | Good to Moderate |
| Room with single-hose portable AC running | 85–160 | Unhealthy |
| Room with dual-hose portable AC + filtered intake | 20–40 | Good to Moderate |
| Room with window AC (sealed installation) | 15–30 | Good |
The single-hose unit increased indoor PM2.5 by 3–5x compared to a sealed room, essentially negating the benefits of keeping windows closed.
Dual-Hose Units: Controlled Outside Air Intake
Dual-hose portable ACs do intentionally pull air from outside — but through a dedicated intake hose that feeds the condenser coil only. This air passes over the condenser to remove heat, then exits through the exhaust hose. It never enters your room or mixes with your breathing air.
Because the dual-hose unit draws condenser air from outside, it doesn't create negative pressure in your room. No negative pressure means no uncontrolled infiltration through building gaps.
The Catch: Hose Seal Quality
Even dual-hose units aren't perfectly sealed. The window bracket, hose connections, and the unit itself can have small air leaks. If the seal between the intake hose and the window bracket is poor, some outdoor air can leak into your room. However, this leakage is typically 5%–10% of what a single-hose unit causes.
How to Protect Indoor Air Quality During Smoke Events
Option 1: Use a Dual-Hose Unit With Filtered Intake (Best)
Fit a MERV-13 or higher filter over the intake hose's outdoor opening. This filters the air the condenser draws from outside, reducing PM2.5 by 85%–95%. Since this air doesn't enter your room anyway, the filter is extra protection against any potential leakage.
Materials needed: MERV-13 furnace filter ($10–$15), duct tape, and a cardboard adapter if the filter doesn't match the hose diameter.
How to do it: Cut the filter to size. Wrap it around the exterior opening of the intake hose, securing with duct tape. Replace the filter when visibly discolored (typically every 1–3 days during heavy smoke).
Option 2: Seal All Gaps Around Single-Hose Setup
If you have a single-hose unit and can't switch to dual-hose, minimize infiltration by sealing as many entry points as possible. Apply foam weatherstripping under doors, seal gaps around the window bracket with foam tape, block electrical outlet gaps on exterior walls with foam gaskets, close all interior doors to reduce the volume of space the AC depressurizes, and run the AC on its lowest setting to reduce exhaust volume.
This won't eliminate infiltration but can reduce it by 40%–60%.
Option 3: Pair With a Portable Air Purifier
Run a portable HEPA air purifier alongside your portable AC. The purifier won't prevent smoky air from entering, but it will filter it once inside. For effective coverage during smoke events, you need a purifier rated for your room size with a CADR of at least 200 for smoke/particles.
Option 4: Use Fan-Only Mode
If smoke is severe (AQI 300+), consider turning off the AC's cooling mode and using fan-only (no compressor). In fan-only mode, no air is exhausted, so no negative pressure is created. You lose cooling, but you preserve indoor air quality. Pair with a portable fan and blackout curtains to manage heat.
Example 1: Northern California Wildfire Response During a 2026 wildfire event, Lisa used a dual-hose Whynter ARC-14S with a MERV-13 filter taped over the intake opening. Outdoor AQI was 280 (Very Unhealthy). Her indoor PM2.5 stayed at 18–25 μg/m³ (Good AQI), measured with a PurpleAir sensor. She maintained 74°F indoor temperature while outdoor temps hit 98°F. Her neighbor with a single-hose unit measured indoor PM2.5 of 120+ μg/m³.
Example 2: Seattle Apartment During Smoke Season Mike had a single-hose portable AC when smoke from Canadian wildfires blanketed Seattle. After seeing his indoor AQI rise to 140 with the AC running, he sealed door gaps with towels, added a DIY Corsi-Rosenthal box fan filter as an air purifier, and reduced the AC to low speed. Indoor AQI dropped to 60 — still not great, but much better. He bought a dual-hose unit for the following season.
Example 3: Commercial Office Server Room An IT company in Sacramento vented a portable AC through a drop ceiling for their server room. During wildfire events, the server room maintained normal air quality because the plenum space above the drop ceiling acted as a buffer. The portable AC exhausted into the plenum, and the building's HVAC system filtered the plenum air.
Recommended Equipment for Smoke Season
| Equipment | Purpose | Cost | Effectiveness |
|---|---|---|---|
| Dual-hose portable AC | Cooling without negative pressure | $400–$800 | Excellent |
| MERV-13 furnace filters | Filter dual-hose intake | $10–$15 each | 85%–95% PM2.5 reduction |
| HEPA air purifier (CADR 200+) | Clean room air directly | $100–$250 | 99%+ PM2.5 reduction |
| Corsi-Rosenthal box fan filter | DIY air purifier | $40–$60 | 70%–90% PM2.5 reduction |
| Foam weatherstripping | Seal door/window gaps | $5–$15 | Reduces infiltration 40%–60% |
| PurpleAir or AirGradient sensor | Monitor indoor PM2.5 | $100–$250 | Real-time AQI feedback |
Key Takeaways
- Single-hose portable ACs cause indoor air quality problems during smoke events by creating negative pressure that pulls outdoor air through building gaps.
- Dual-hose portable ACs are significantly better for smoke events because they don't create negative pressure.
- Filter the dual-hose intake with MERV-13 material for extra protection during smoke.
- A HEPA air purifier is essential if you must use a single-hose unit during poor AQI.
- Indoor PM2.5 can be 3–10x higher with a single-hose AC running during smoke vs. a sealed room.
- Fan-only mode creates no negative pressure — use it when AQI is extreme.
- Monitor your indoor air with a PM2.5 sensor to make informed decisions.
Frequently Asked Questions
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