Scented Candles Can Alter Your Home's Microbiome?

Most people think of scented candles as harmless—maybe even beneficial. They make a room feel calm, cozy, and inviting. Yet we are now learning that there's more to this story, not only because of the pollution they produce, but also because of the changes to the microbiome of our homes. A recent study published in Nature Scientific Reports takes a deeper look at what actually happens to indoor air when a scented candle is lit. 

To start, it's important to know that when you burn a candle, you’re creating combustion. That's the physical reaction of fuel with oxygen and fire that creates heat. And combustion—whether from a car engine, a gas stove, or a candle—produces particulate matter (PM). These are tiny particles suspended in the air. Depending on their size, PM can have different effects on our physiology. The worst forms for health are thought to be 2.5 microns or smaller (PM 2.5)

In this study, researchers measured three types of particles: PM10 (larger particles), PM2.5 (fine particles), and PM1 (ultrafine particles). Within just five minutes of lighting a candle, PM10 levels near the flame rose by over 50% compared to baseline. But the more important story wasn’t what happened right next to the candle—it was what happened across the room. The smallest particles didn’t just rise briefly—they traveled.

As the candle burned, these fine and ultrafine particles spread throughout the indoor space, remaining elevated even at distances of 3 to 6 meters away. Unlike larger particles, which tend to settle quickly, these smaller particles stay airborne longer and disperse more widely. In fact, the study showed that the concentration of ultrafine particles remained consistently high even far from the source, suggesting that once they’re in your air, they linger. 

This matters because smaller particles behave differently in the body. Their size allows them to bypass many of the body’s natural defenses, reaching deeper into tissues and interacting more directly with biological systems. But perhaps the most interesting finding from this study wasn’t just about particles. It was about biology.

The researchers didn’t just measure pollution—they analyzed the indoor microbiome. In other words, they looked at how candle burning changes the living ecosystem of bacteria and biological particles floating in your air (yes, there are bacteria and other microbes in your air!)

Before the candle was lit, the indoor air contained a relatively stable mix of airborne bacteria. But after burning, the microbial landscape shifted significantly. Most notably, there was a sharp rise in something called bacterial extracellular vesicles (EVs).

These EVs are microscopic particles released by bacteria. Think of them as tiny packages filled with biological signals—proteins, genetic material, and other molecules—that can interact with human cells. Unlike whole bacteria, these vesicles are much smaller, more mobile, and more likely to remain suspended in the air.

After candle use, the study found a significant increase in these EVs. Exactly why, is unclear. But why does this matter? EVs can interact with your body in ways we’re only beginning to understand. Due to their size, they can enter the bloodstream, communicate with immune cells, and potentially influence inflammation and other biological processes. The key takeaway isn’t that candles are uniquely dangerous—it’s that indoor air is far more biologically active than we tend to assume.

What’s likely happening is that the heat and byproducts of combustion create a new environment—one that changes how microbes grow, die, and release these vesicles. In some cases, existing bacteria may become more active. In others, they may break down, releasing additional biological material into the air.

Another important insight from this study is how long these effects last. While larger particles dropped relatively quickly after the candle was extinguished, the smallest particles—and the microbial changes—persisted. This reinforces a broader theme in air quality research: what you do indoors doesn’t stay localized or temporary. It spreads, lingers, and accumulates.

If you’re going to use candles, a few simple steps can help reduce impact: improve ventilation, avoid prolonged burning in small enclosed spaces, and be mindful of frequency. Even spacing out use can make a difference in cumulative exposure.