What’s the difference between indoor and outdoor allergies?

The allergy aisle can be confusing.

Despite affecting some 50 million Americans, allergies aren’t super well understood. The sparks that ignite your immune system can range from sunlight to onions, and symptoms of an attack are just as varied. For that reason, we’re spending several weeks writing about allergies—what they are, how they manifest, and how we can find relief. This is PopSci’s Allergic Reaction.

The allergy aisle is filled with products purporting to be your springtime savior. “All day relief!” “Non-drowsy!” “Indoor and outdoor!” But wait, hang on—are indoor and outdoor allergens really that different?

“I assure you, your immune system doesn’t discriminate whether you are sensitive to a grass, tree or ragweed pollen, they may all trigger allergy misery,” says Clifford Bassett, the medical director of Allergy & Asthma Care of New York. That also goes for whatever allergens lurk inside your house, he explains, including both your pets and your unseen dust mite roommates. It’s just that some allergens are more prevalent outdoors, while others stay inside.

But that’s not to say you’ll react to them all the same way. “It’s fair to say that each allergen is uniquely different on its own,” says Sarena Sawlani, medical director of Chicago Allergy & Asthma. “Mold, dust, and pollen for example can each trigger completely different allergic responses.” Those responses all rely on the same basic mechanism inside our bodies—namely, histamines (more on that here)—it’s just that each person will react to the allergens differently.

Maybe dog dander brings tears to your eyes. Maybe certain pollens makes you sneeze. These aren’t fundamental differences in how these allergens act inside your body, but a change in how your immune system reacts.

Perhaps more importantly, the drugs you’re looking at in the pharmacy aisle aren’t treating the allergens differently. Antihistamines, which make up the majority of the pill-based solutions Americans take, prevent histamines from binding to the receptor on cells in your mucus membranes that triggers inflammation (check out our other Allergic Reaction article for a more in-depth look at histamines!). Corticosteroids—the active ingredient in those trusty nasal sprays—prevent histamines from even releasing in the first place by blocking the influx of immune cells to your mucus membranes. They’re slightly different mechanisms, but they both block the same reaction.

Whether or not a particular drug handles your symptoms is more dependent on your body than on the allergen. People react differently to each individual medication, so unfortunately finding the right one is a matter of trial and error. You may even want a combination of meds.

The most basic way to figure out whether your allergies are coming from inside your house or outside is simply to pay attention to when you get your symptoms. If you have them year round, you’ve probably got something irritating you inside your home. If they peak during the spring and summer, outdoor allergens are probably at least a partial factor. But that may not be the full picture. “Air pollutants can synergize with pollen in the air to create a double whammy effect and magnify and further worsen seasonal symptoms,” explains Bassett. He says the only real way to know exactly what’s bothering your immune system is to go to an allergist and get tested. Though not everyone can afford that option, it is an excellent way to better understand your allergies and figure out a way to handle them. You may even be a good candidate for allergy shots, which can help dampen reactions by teaching your immune system not to treat them as dangerous invaders. And if you’re not, at least you can talk to someone about how to best handle your reaction. Your allergist may recommend certain kinds of pillows or nasal irrigation—things you might not have considered.

Barring that, don’t stress about which allergy medication you take. If you think it works for you, stick with it. Indoor, outdoor, pet-related, dust mite-induced—it doesn’t matter if your symptoms are under control.