The Indoor Air Paradox: Rising PM2.5 Pollution Levels

Introduction

We typically perceive indoor air as a refuge, safer and cleaner, away from the outdoor smog and dust. The reality is much less reassuring: PM_2.5 often builds up to higher concentrations indoors than outdoors. The fact that one spends approximately 80–90% of their time indoors in homes, offices, and schools means that indoor exposure gradually becomes the majority contributor to our total particulate breathing dose.

In today's world, this is particularly important. Buildings are designed to be tighter for energy efficiency purposes, while outdoor pollution acts as a natural indoor pollutant, with additional contributions from smoke produced by cooking, burning candles, or using any heating appliance. Understanding how indoor sources interact and why PM_2.5 levels increase indoors is crucial to understanding why the air in many homes, behind closed doors, is some of the most polluted air a person may breathe.

What Makes PM_2.5 Indoors So High? The Core Drivers

Indoor PM_2.5 levels have increased rapidly since many of our commonly carried out indoor activities directly emit fine particulate matter into the air. Cooking, for example, is a significant source; frying, roasting, or tempering foods can produce short-term spikes, especially in homes burning biomass or kerosene, but these burning behaviors contribute to the baseline. Incense, candles, and mosquito coils emit considerable particulate matter; indeed, some evidence suggests that the PM_2.5 produced from a single mosquito coil is equivalent to the total particle load from multiple burning cigarettes. In colder climates, wood stoves and space heaters contribute in the same manner.

Outdoor pollution can enter indoors more easily than most people realize. Windows, unsealed doors, and ventilation pathways always allow outdoor air to infiltrate. In places like North India, for example, indoor air quality often mirrors the outdoor conditions during the winter months. Wildfire events in California produced similar evidence that indoor air can exceed 100 µg/m³, even in closed homes. The degree of infiltration can vary, but a significant portion of outdoor PM_2.5 typically ends up indoors.

Generally, once PM_2.5 enters an indoor space, it becomes deposited and remains for some time. Poor ventilation slows down the PM_2.5 dispersion indoors, and in sealed AC rooms, indoor PM_2.5 stays longer in poorly ventilated spaces.

Together, these factors show why indoor PM_2.5 often becomes higher than outdoor levels: strong indoor sources, constant infiltration, slow removal, and repeated daily activities all contribute to a heavier, more concentrated air environment inside the home.

When Indoors Exceed Outdoors: Evidence From Studies

It is a common occurrence to assume that outdoor air is worse than indoor air. However, indoor PM_2.5 can be higher than outdoor PM_2.5, depending on household behaviors, activities, and the characteristics of the built environment, such as ventilation and outdoor air quality. Studies from the WHO and the NIH indicate that indoor concentrations can be two to five times greater than outdoor air, especially during activities such as cooking, heating, and burning incense.

In parts of California where wildfires had a direct impact, outdoor PM_2.5 levels rapidly increased. However, most homes with low-grade filtration often showed indoor PM_2.5 levels at 60 to 70% of outdoor levels and fluctuated to be higher than outdoor levels if there were indoor emissions occurring simultaneously. In rural South Asia, homes that utilize biomass fuels for cooking and heating report indoor concentrations of PM_2.5 ranging from 300 to 1500 µg/m³, whereas outdoor levels are typically between 150 and 300 µg/m³. These examples demonstrate that indoor air conditions differ from outdoor air conditions, as they are influenced by multiple sources, including indoor-specific sources, building mitigation factors, and ventilation practices.

Why Indoor Exposure Matters More Than Outdoor Exposure

Indoor exposure often matters more than outdoor air simply because we spend most of our time indoors. Even if outdoor PM_2.5 levels are higher, the cumulative dose we inhale is still largely determined by indoor concentrations. This means that our daily habits, such as cooking, heating, and using scented products indoors, have a more significant impact than we might imagine. Furthermore, sensitive groups, such as children, the elderly, or individuals working from home, often spend more time indoors, which also increases their potential exposure to particulates.

Indoor PM_2.5 is a concept often perceived as a complex mixture of organic aerosols, combustion products, or ultra-fines that may behave differently from outdoor PM_2.5, which can penetrate deeply into the lungs. For example, a person living in Delhi can expose themselves to ambient PM_2.5 levels of 250 µg/m³ outdoors, but if that person spends three hours cooking in their kitchen, experiencing short-term averages of 500-1000 µg/m³ concentrations, and then the remainder of their time indoors, they likely have a higher indoor cumulative exposure than indoors.

How to Reduce Indoor PM_2.5 Without Overcomplicating Life

It is surprisingly simple to reduce PM_2.5 exposures indoors. You can start simply by limiting the cooking and burning of materials that will create high emissions during cooking.

• Cover pans with lids while they are frying. Use induction or electric cooking instead of gas. Also, limit the burning of incense, candles, and mosquito coils.

• Ventilate judiciously by opening windows or cross-ventilating when outdoor pollution is low. Do not vent if there are ongoing wildfires in California or smog episodes.  Even 15-30 minutes of targeted ventilation to refresh air is sufficient.

• Carefully manage infiltration by keeping windows and doors shut during periods of peak outdoor pollution. In addition, routine housekeeping, including HEPA vacuuming and damp dusting, as well as limiting dust accumulation on carpets, can all help mitigate indoor PM_2.5 particulate levels.

• Finally, simple filtration can help achieve reductions, like the use of HEPA filters. Combined, smart cooking, proper ventilation, normal filtration, and good cleaning habits will reduce PM_2.5 exposure without significantly altering your daily life.

Conclusion:

Indoor PM_2.5 is not a trace that vanishes the moment we close our doors; it generally lingers, accumulating from the food we cook, our burning habits, and the air that comes in from outside. The problem is not something distant or invisible; it is shaped by what we do every day. Understanding that easy habits, improving ventilation, smart cleaning, and clean cooking can meaningfully reduce our exposure makes the problem less daunting. At times, knowledge is the most significant filter that we have.

A Collective Responsibility

In this modern age, air quality is everyone's responsibility. Each action we take can contribute to a healthier planet. Planting trees, choosing public transportation, or supporting local clean air initiatives can make a difference. Awareness is the first step toward change.

Join us in advocating for cleaner air. Together, we can help nature regain its voice.

Let’s work together to ensure our world thrives in harmony with nature.