Positive vs Negative vs Neutral Pressure Ventilation: Which Is Best for Homes in Polluted Cities?

By Barun Aggarwal, Founder & CEO, BreatheEasy. Technically reviewed by Izhar Ahmed, Chief Technology Officer, BreatheEasy. Published 23 September 2026. Last updated 23 September 2026.

•       Every ventilation system leaves your home at negative, neutral or positive pressure, and that decides which way air leaks through gaps around windows and doors.

•       Negative pressure pulls unfiltered outdoor air in through every gap. Neutral pressure does not control which way air leaks. Positive pressure pushes clean air out through the gaps instead.

•       In a city like Delhi, where outdoor PM2.5 averaged 97 µg/m³ in 2025, positive pressure with HEPA-filtered supply air is the strongest way to stop leakage from undoing your filtration.

•       Positive pressure has limits: it needs a reasonably sealed home, it costs energy, and it does not remove gases without a gas filter.

The short answer

For homes in polluted cities, positive pressure is usually the best choice, because it makes air leak out of your home rather than in. Negative pressure, created by exhaust fans and kitchen chimneys, pulls polluted outdoor air in through every gap. Neutral pressure, from balanced systems like ERVs, saves energy but does not control leakage. In colder or cleaner-air climates, the answer can be different.

What does "pressure" mean in home ventilation?

It means whether the air inside your home is at a slightly higher or lower pressure than the air outside. The differences are tiny, often just a few pascals (Pa), far too small to feel. But air always moves from higher pressure to lower pressure, so even a few pascals decide which way air flows through every gap in your home.

Modern apartments are built well sealed, which is why CO₂ builds up once the windows are shut, and why they need a fresh air system in the first place. But every home still has small openings: under doors, around window frames, through AC pipe holes, electrical conduits and exhaust fan openings. When indoor pressure is lower, outdoor air rushes in through them unfiltered. When indoor pressure is higher, indoor air leaks out through them instead.

The three types at a glance

What is negative pressure?

Negative pressure happens when more air is pulled out of your home than is supplied. Kitchen chimneys and bathroom exhaust fans do this every time they run. Exhaust-only ventilation systems do it all the time.

The US DOE's Building America programme lists the risks. "Depressurizing the home draws in air from outside the home. The key question is: where does this makeup air come from?" (Aldrich, 2014). In Delhi, the answer is usually through window gaps, straight from polluted outdoor air. The same guidance warns that depressurising a home "can interfere with natural-draft combustion appliances" and "can exacerbate introduction of radon or soil gas."

What is neutral (balanced) pressure?

Neutral pressure happens when a system supplies and removes equal amounts of air. ERVs (energy recovery ventilators) and HRVs (heat recovery ventilators) work this way, and the DOE notes they "don't induce a large pressure (either positive or negative) on the building" (Aldrich, 2014).

The advantage is energy. Balanced systems can recover 40 to 80 percent of the heating or cooling energy in the air they exhaust (AHRI).

The drawback is that neutral pressure does not control which way air leaks. In Delhi, that means some unfiltered outdoor air can still get in.

What is positive pressure?

Positive pressure happens when more clean air is supplied than is removed, so air leaks out through gaps instead of in. Supply-only fresh air systems, such as VaaYoo PRO1, create it by pushing filtered outdoor air in and letting it leave through the home's natural gaps and exhaust fans.

The US DOE notes that "a 'supply-only' ventilation strategy is commonly recommended for hot, humid climates" (DOE Building America, 2009). The same guidance warns about the opposite case: in cold climates, "pressurizing building interiors may force air through wall assemblies, potentially leading to moisture condensation."

Why positive pressure matters most in polluted cities

In a polluted city, the air leaking in through gaps is often the biggest source of indoor pollution. Positive pressure is the only one of the three that stops it.

Studies of Delhi homes show how much gets in. In winter, for every 10,000 particles added outdoors, about 3,000 more appeared indoors, and "indoor air quality while using an air purifier is frequently worse than in cities with moderate pollution" (Vyas et al., PLoS ONE, 2016). EPIC India found that high-income Delhi households were 13 times more likely to own air purifiers, yet their indoor pollution was only 10 percent lower (EPIC India). Room purifiers clean the air, but leakage keeps bringing more pollution in. We explain this in more detail in why clean indoor air needs more than air purifiers.

Hospitals solve the same problem with positive pressure. The US CDC's standard for protective rooms, used for patients with weak immune systems, calls for HEPA-filtered supply air, positive pressure relative to the corridor of more than 2.5 Pa, and well-sealed rooms (CDC, Isolation Precautions, Appendix A).

What does the research show?

•       Homes in New Zealand, 2026: positive pressure ventilation "reduced indoor PM2.5 by 38-62%" across 24 homes, and indoor-to-outdoor ratios fell below 1.0 in most of them (Hernandez et al., Building and Environment, 2026). Outdoor pollution there is far lower than in Delhi, so this shows the effect of pressure, not Delhi-level protection.

•       A school in Finland, 2018: classrooms were pressurised to a target of 5 to 7 Pa. Mean PM2.5 fell from 1.80 to 1.27 µg/m³ and TVOCs fell 21 to 55 percent. But one classroom stayed above 0 Pa only 45 percent of the time (Vornanen-Winqvist et al., IJERPH, 2018). A room that cannot hold its pressure loses much of the benefit.

•       Offices in China, India, the UK and the US, 2021: a 10 µg/m³ rise in outdoor PM2.5 was linked to a 19.9 percent rise indoors during work hours, and buildings with high-grade (MERV 15 or better) filters had 39.4 percent lower indoor PM2.5 (Jones et al., Building and Environment, 2021). Filtration and controlled airflow both matter.

In our own 12-month monitoring of a positive-pressure VaaYoo installation in a 1,500 sq ft Delhi NCR apartment in 2025, indoor PM2.5 averaged 8 to 10 µg/m³ across the year, and stayed at or below 18 µg/m³ even when outdoor levels peaked near 420 µg/m³ in November. This is one home, measured with an IQAir AirVisual Pro monitor, not a controlled study. See the full numbers in our ERV vs VaaYoo comparison.

What are the limits of positive pressure?

Positive pressure has three limits:

1.    Your home must be reasonably sealed. A very leaky home, or a window left open, lets the pressure escape. The Finnish school study shows how quickly a leaky room loses the benefit.

2.    It does not remove gases on its own. HEPA filters catch particles only. The US EPA states that "air-cleaning devices designed only to remove particles are incapable of controlling gases" (EPA, 2018). For NO₂ and ozone from traffic and generators, you need a gas-phase filter, such as activated carbon, chosen for those gases.

3.    It costs energy. Every litre of outdoor air must be cooled, dried or heated, and a supply-only system recovers none of that energy. An ERV recovers 40 to 80 percent (AHRI). Our ERV vs VaaYoo guide explains this trade-off.

For a professional check of your home's air, BreatheEasy offers indoor air quality testing in Delhi.

Frequently asked questions

Should a house have positive or negative pressure?

In a polluted city, slightly positive pressure is usually better, because it stops unfiltered outdoor air from leaking in.

Is positive pressure good for a house?

Yes, in most polluted and hot-humid cities. Positive pressure pushes clean, filtered air out through gaps instead of letting polluted air in. The US DOE recommends supply-only ventilation, which creates positive pressure, for hot, humid climates. Its main downsides are higher energy use and possible condensation in very cold climates.

Does positive pressure cause mould or condensation?

It can in very cold climates, where warm, moist indoor air pushed into cold walls may condense. The US DOE warns of this risk.

Does an ERV create positive pressure?

No. An ERV is a balanced system, supplying and removing equal amounts of air, so it keeps your home close to neutral pressure.

How much positive pressure does a home need?

There is no single figure for homes. For comparison, US hospital protective rooms must stay more than 2.5 Pa above the corridor (CDC), and a Finnish school study targeted 5 to 7 Pa (Vornanen-Winqvist et al., 2018).

Does positive pressure keep out gases like NO₂ and ozone?

It keeps unfiltered outdoor air from leaking in, which reduces gases entering through gaps. But the fresh air it supplies still carries any gases that pass its filters. HEPA filters do not remove gases, so to cut NO₂ or ozone you also need a gas-phase filter such as activated carbon.

Sources

•       Aldrich, Measure Guideline: Selecting Ventilation Systems for Existing Homes, US DOE Building America, 2014. https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/measure_guide_ventilation_systems.pdf

•       US DOE Building America, Ventilating Your Home, 2009. https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/ventilation_guide.pdf

•       US DOE Building Science Education, Whole-house ventilation. https://bsesc.energy.gov/energy-basics/hvac-whole-house-ventilation-0

•       US CDC, Isolation Precautions, Appendix A, Table 5. https://www.cdc.gov/infection-control/hcp/isolation-precautions/appendix-a-table-5.html

•       Hernandez et al., Building and Environment, 2026. https://doi.org/10.1016/j.buildenv.2026.114346

•       Vornanen-Winqvist et al., IJERPH, 2018. https://pmc.ncbi.nlm.nih.gov/articles/PMC5858299/

•       Jones et al., Building and Environment, 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC8336933/

•       Vyas et al., PLoS ONE, 2016. https://pmc.ncbi.nlm.nih.gov/articles/PMC5158316/

•       EPIC India, Delhi homes record poor indoor air quality. https://epic.uchicago.edu/news/delhi-homes-record-poor-indoor-air-quality-low-willingness-for-demanding-clean-air/

•       US EPA, Residential Air Cleaners: A Technical Summary, 3rd edition, 2018. https://www.epa.gov/sites/default/files/2018-07/documents/residential_air_cleaners_-_a_technical_summary_3rd_edition.pdf

•       AHRI, Air-to-Air Energy Recovery Ventilators. https://www.ahrinet.org/scholarships-education/education/contractors-and-specifiers/hvacr-equipmentcomponents/air-air-energy-recovery-ventilators-ervs

•       MoEFCC via PIB, Delhi air quality 2025, published 2 January 2026. https://www.pib.gov.in/PressReleasePage.aspx?PRID=2210935

About the author

Barun Aggarwal is the Founder & CEO of BreatheEasy. He co-authored India's indoor air quality standard with ISHRAE, won the Asian Development Bank's Global Challenge (US$300,000), and is a WELL AP and RESET AP. He is the author of How to Grow Fresh Air and a co-founder of the clean-air NGO Care for Air. Barun on LinkedIn

Technical review: Izhar Ahmed is Chief Technology Officer at BreatheEasy. He is a mechanical engineer with more than 15 years in HVAC and indoor air quality, and has delivered more than 1,000 residential and 300 commercial projects. Izhar on LinkedIn