The country's heat wave has become a major public health disaster, and the government is giving potential victims inadequate advice, Ralph Nader's health director maintained yesterday.

When the mercury tops 95, said Dr. Sidney Wolfe, head of the Public Citizen Health Research Group, almost everyone who lacks air conditioning should drink about 1 1/2 times as much liquid daily as thirst dictates, which means at least a gallon a day.

U.S. Surgeon General Julius Richmond advised recently that people use common sense by drinking plenty of liquids. Common sense won't do, Wolfe said. The toll in heat-related deaths is higher than in many so-called epidemics, he said, and "deficiencies" in Richmond's advice "might result in preventable deaths and illness."

In a letter prompted by some state health officials, who say they want more advice, and drawing on latest findings and expert opinion, Wolfe wrote Richmond that:

Anyone not on a salt or fluid-restricted diet should drink at least a gallon of liquid daily, unless that person is in an air-conditioned environment. The very large, the overweight and those exercising in the heat need even more.

When it is hot, people should rest their over-worked sweat glands by bathing or showering, than air-drying rather than toweling dry, by wearing moistened clothese and by wrapping up incool, wet towels.

These rules should be followed particularly by the elderly, those with heart disease and those taking drugs that reduce sweating.

Those drugs are:

Diuretics or "water pills," such as Diamox (acetazolamide), Diuril Chlorothiazide), Enduron (methylclothia zide), Esidrex, Hydrodiuril (hydrochlorothiazide), Hygroton (chlorthalidone), Diazide (triampterene, hydrochlorothiazide), and Lasix (furosamide).

Phenothiazine tranquilizers, such as Thorazine (chlorpromazine), Stelazine (trifluorperazine hydrochloride), Phenergan (promethazine), Temaril (trimeprazine), and Vesprin (triflupromazine).

Ulcer and gastrointestinal drugs such as Pro-Banthine (propatholine), Chardonna-2 (belladonna), Bentyl (dicyclomine), Donnatal (atropine, hyoscine, hyocyamine) or nonbrand-name atropine, belladonna, scopolamine, clinidine and osopropamide.

Wolfe, who once did heat research at the National Institutes of Health, urged the government to get more information to health officials and the public, partly by mailing it with Social Security checks.

The surgeon general on July 18 also told people to stay indoors and out of the sun, and to use alcohol, including beer and wine, sparingly if at all. He told the elderly or ill to use fans, icebags or wet towels (taking care not to suffer electric shock by touching electric appliances while wet), to wear light, loose cloths and to avoid exertion, salt tablets, over-salty drinks and all alcohol.

Richmond yesterday made no direct response to Wolfe, but agreed that the over-heated should drink more liquids, even if they don't feel thirsty. He urged everyone to avoid overeating and exertion. He urged those with heart disease to find air-conditioned shelter, and those on medically regulated salt and water diets to have these adjusted "on a carefully controlled individual basis by your physician."

Wolf made a few more suggestions: Avoid or drink less coffee and tea, because they, like alcohol, may cause fluid loss. "if you can, take your temperature and weigh yourself everyday," and if your temperature rises or weight falls, get help. "Stay in daily contact with other people."