Q.Collect information about DDT as a persistent pesticide.

Several individually important polyhalogen compounds combine everyday industrial or medical usefulness with documented health or environmental hazards. Dichloromethane (CH2Cl2): a solvent and aerosol propellant; causes dizziness/fatigue/nausea/headache/weakness on overexposure and damages the cornea on eye contact. Chloroform (CHCl3): used to make the refrigerant R-22, as an extraction solvent, and as a laboratory source of dichlorocarbene; depresses the central nervous system, and slowly converts to poisonous phosgene on exposure to air and light, so it is stored in dark, airtight bottles. Carbon tetrachloride (CCl4): a solvent for oils/fats/resins, a chlorine source, and a cleaning agent; highly toxic to the liver, suspected carcinogen, and a greenhouse gas. Iodoform (CHI3): a limited-use wound antiseptic/healing agent; irritates skin/eyes and can cause respiratory irritation and CNS depression. The Freons (chlorofluorocarbons, e.g. Freon-12/dichlorodifluoromethane, R-22, R-11): used as refrigerants, aerosol propellants, and foam-blowing agents; they deplete the stratospheric ozone layer, because UV light releases a chlorine atom from the …