kinds: particulate (e.g. alpha and beta radiation, neutrons, positrons) and electromagnetic (e.g. gamma radiation and x-rays). Alpha particles are made up of two neutrons and two protons and carry a 2+ charge. They do not penetrate tissues well, but produce intense tissue ionization. Beta particles are electrons or positrons, carrying only a single negative charge. They can penetrate several millimeters into tissues, but induce only moderate ionization. Gamma rays and x-rays do not carry a charge, and have no mass, so can penetrate deeper into tissues, creating moderate ion-ization. Gamma rays have higher energy than x-rays. The deleterious effects of ionizing radiation on living tissues is related to the high water content of tissues. The radiation generates highly reactive H and OH radicals that can damage DNA and proteins. See also radical.
ionosphere The part of the Earth's atmosphere that extends from about 50 km to the outer edge of the atmosphere at around 500 km, in which ionization of atmospheric gases is sufficient to affect the propagation of radio waves. The amount of ionization varies with the time of day, seasons, and solar cycle.
IPC See integrated pollution control.
IPCC See Intergovernmental Panel on Climate Change.
IPM See integrated pest management.
iron (Fe) A metallic element that makes up some 5% of the Earth's crust. It is an essential nutrient for animal and plant growth. Iron is a constituent of the red blood pigment hemoglobin and plays an important part in many metabolic activities.
iron pan See hardpan.
irradiation The exposure of an object, organism, or chemical to ionizing radia tion such as high-energy particles or gamma rays or x-rays.
irrigation The supplying of water or wastewater to land by artificial means to enable plants to grow.
irruption A sudden unpredictable increase in the size of a population, leading to the emigration of large numbers of individuals.
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