Intraocular pressure refers to the tissue pressure within the human eye and is typically caused by the continual drainage and production of aqueous humors, states MedicineNet. The eye pressure is usually shown as millimeters of mercury (mm Hg).
The standard eye pressure should measure less than 21 mm Hg; however, this measurement varies with populations. Ocular hypertension occurs when the eye pressure increases. Individuals with ocular hypertension have a higher risk of developing glaucoma. Intraocular pressure slowly increases with age. According to WebMD, 3 to 5 percent of people over 40 are affected by ocular hypertension. An eye pressure test can be carried out with a tonometer to measure the fluid pressure within the eye.