A sinus is an air-filled cavity inside the bones of your skull, connected to your nasal passages. You have four pairs of these cavities, called paranasal sinuses, and they sit in the bones around your nose, eyes, and forehead. Together, they lighten the weight of your skull, produce mucus, warm and humidify the air you breathe, and act as resonating chambers that shape the quality of your voice.
Where Your Four Sinus Pairs Are Located
Each pair of sinuses is named after the bone it sits in. The maxillary sinuses are the largest. They’re pyramid-shaped cavities inside your cheekbones, sitting just beneath your eyes on either side of your nose. These are also the first to start forming, beginning in the third month of fetal development, though they don’t reach full size until your late teens or even your twenties.
Your frontal sinuses sit in the bone of your forehead, just above your eyebrows. They’re the last sinuses to fully develop, typically not reaching adult size until age 20 to 25. The ethmoid sinuses are a cluster of small air cells located between your eyes, behind the bridge of your nose. They tend to reach their full size earliest, around age 12 to 15. Finally, the sphenoid sinuses sit deep inside your skull, behind your eyes. Of all four pairs, these are the farthest inside your head, and they complete most of their growth by age 14 to 16.
All four pairs open into your nasal cavity through small passageways called ostia. These openings are how air enters the sinuses and how mucus drains out. When those passageways get blocked, problems start.
How Sinuses Keep Themselves Clean
The inside walls of your sinuses are lined with a thin tissue that produces mucus. This mucus layer traps dust, bacteria, and other particles that make it into the cavities. But producing mucus is only half the job. The lining is also covered with millions of tiny hair-like structures called cilia, each about 7 micrometers tall, that beat in coordinated waves to sweep the mucus (along with whatever it has trapped) toward the drainage openings and out into your nasal cavity.
Each cilium beats at the same frequency as its neighbors but slightly out of phase, creating a ripple effect across the surface, like a wave moving through a stadium crowd. This wave pushes the mucus layer steadily toward the exit. For this system to work, the thin fluid layer beneath the mucus needs to stay properly hydrated. If it dries out, the mucus collapses onto the cilia and they get stuck, which stalls the whole cleaning process and can set the stage for infection.
What Happens When Sinuses Get Blocked
Sinusitis, or sinus inflammation, is one of the most common health complaints. CDC data from 2018 found that roughly 28.9 million American adults had been diagnosed with sinusitis that year, about 11.6% of the adult population. It happens when the sinus drainage pathways swell shut, trapping mucus inside the cavity. Bacteria or viruses can then multiply in that stagnant fluid, causing infection.
The most frequent triggers are viral upper respiratory infections (common colds), allergies, and structural issues like a deviated septum. A deviated septum narrows one side of the nasal cavity, which can obstruct airflow and block sinus drainage. Research has found that about half of patients with chronic sinus inflammation also have some degree of septal deviation, and certain types of deviation that affect the nasal valve area appear more likely to contribute to the problem.
Sinus Pain vs. Other Headaches
True sinus pain tends to produce a steady, mild-to-moderate pressure across your face, at the bridge of your nose, or in your cheeks. It often comes with nasal congestion and postnasal drip, can last many hours, and is frequently seasonal, particularly in people with allergies. That facial location is what distinguishes it from other common headache types.
A tension headache, by comparison, creates a band-like pressure across the forehead or the back of the head, without the congestion or facial focus. A migraine involves moderate to severe throbbing, usually on one side, and typically brings nausea and sensitivity to light and sound. Many people who assume they have a “sinus headache” actually have a migraine, because migraines can cause pain around the eyes and forehead. The key differentiator is congestion: if you don’t have thick nasal discharge or blockage alongside the pain, a sinus problem is less likely the cause.
How Sinus Problems Are Treated
Most acute sinus infections are viral and clear up on their own within 7 to 10 days. During that window, the main goal is to keep mucus moving. Saline nasal rinses help thin the mucus and flush out irritants. Staying well hydrated supports the fluid layer that keeps cilia beating. Steam from a hot shower or a bowl of hot water can temporarily relieve congestion by loosening thick mucus.
Antibiotics only help when the infection is bacterial, which is more likely if symptoms last longer than 10 days, get significantly worse after initial improvement, or include a high fever with thick, discolored discharge. For people with chronic sinusitis (symptoms lasting 12 weeks or longer), treatment may involve nasal corticosteroid sprays to reduce swelling, allergy management if that’s an underlying trigger, or in some cases surgery to widen the blocked drainage pathways. The goal of any intervention is the same: restore the natural drainage and airflow that keep sinuses healthy.