inflammation, flame, blaze
炎 centers on intense, rising fire: a flame or blaze, and by extension the heat and redness of inflammation.
炎 is formed by doubling 火 (fire), visually emphasizing a large or rising flame. This reduplication of fire to express intense burning is a well-established semantic formation.
Two fires stacked on top of each other create a towering blaze. The double fire makes the flame bigger and hotter, like an inflammation.
For エン, picture an enormous blaze at the end of a tunnel: end -> エン, and the double fire lights up the exit.
flame; blaze
going up in flames; destruction by fire (esp. of a large building)
inflammation; irritation
pneumonia
flame; blaze
hepatitis
heat shimmer; heat haze; shimmering of hot air
under the blazing sun; in sweltering heat
rhinitis; coryza; nasal inflammation
Molotov cocktail; petrol bomb; gasoline bomb
tenosynovitis
conjunctivitis
brain inflammation; encephalitis; cerebritis
bronchitis
gastritis; gastric catarrh
otitis media; middle-ear infection
appendicitis
anti-inflammatory (substance)
pancreatitis
flames produced by an explosion; explosive flames