Q. A teen-age boy in our neighborhood confided to me that the current fad among his friends is sniffing a chemical called alkyl nitrate. He said this substance produced a harmless high and was safe because it was sold over the counter by the brand name of "Rush" or "Thrust."

I don't want to betray my little friend to his parents, but I'm also frightened that he may be doing damage to himself in the way kids who sniffed glue did 10 and 20 years ago. Can you give us both a verdict on this substance? A. Glue is sold over the counter, but that doesn't mean it's safe. Sadly, the inhalation of glue and other dangerous substances is not a thing of the past.

Alkyl nitrate, also called butyl or isobutyl nitrate, is a chemical similar to the drug amyl nitrate and the other nitrates used to treat the cardiac chest pain known as angina. Although the FDA does not consider butyl nitrate a drug, inhaling it produces effects similar to those of the nitrate class of drugs. These include dilation of blood vessels and a drop in blood pressure.

When sniffed to excess, butyl nitrate causes a throbbing headache, nausea, vomiting, stomach cramps, rapid heart beat and low blood pressure. In rare cases, prolonged inhalation can damage the oxygen-carrying hemoglobin molecule in your blood, robbing your body of oxygen. Sniffing can make you pass out, and some people consider the sensation of almost passing out a "high." Some people also sniff this substance as an aphrodisiac.

It's true that you can buy various nitrates without a prescription. They're sold thinly disguised as "air fresheners" or "room deodorizers" in novelty stores and what are commonly known as head shops. In some cases, the ingredients of these products aren't even listed on the container. Besides the two you mention, brand names include Bang, Bolt, Climax, Hardware, HiBall, Locker Room, Mama Poppers, Oz, Quicksilver and Satan's Scent.

Sniffing fume-producing substances is a significant health threat to adolescents, with brain damage and death among the possible outcomes. Substances abused in this way include gasoline, aerosol sprays, glue solvents, cleaning agents, lighter fluid, paints, typewriter correction fluid and other volatile commercial products.

In response to this worldwide problem, the manufacturing industry has formed the Solvent Abuse Foundation for Education, which funds school and public awareness programs sponsored by the Center for Continuing Education.

My verdict on sniffing? Don't do it. Q. I recently used a combination of Clorox bleach and Ajax with ammonia to wash my plastic shower curtain. Almost immediately my eyes started to sting. I had to leave the bathroom several times to get fresh air, choking all the while. Fumes filled the house. What reaction took place, and could these fumes result in any serious problem? A. You stumbled across a little household chemistry, triggering a reaction that's irritating though not usually serious.

Bleach contains sodium hypochlorite, a chemical that in itself is mildly irritating. When mixed with ammonia, it creates chloramine gas, a more irritating compound that burns your eyes and causes coughing, choking, tearing and sometimes nausea and vomiting.

Most reactions from household exposures like yours are mild and short-lived. Treatment includes getting away from the fumes, getting fresh air, rinsing your eyes if they're burning and washing any parts of your body that came into contact with the mixture.

A more serious reaction occurs when you mix bleach with acids, such as those found in toilet bowl cleaners. Chlorine gas forms, which is more toxic than chloramine. Exposure produces coughing, choking, tearing, rapid heart beat, a burning sensation in your chest and sometimes serious lung problems if the exposure is severe. Occasionally vomiting, sweating and headache occur.

Treatment of chlorine gas exposure is similar to that for chloramine, except for the higher risk of lung problems requiring medical attention. No toxic gases form when bleach is mixed with detergents that don't contain ammonia.

A good resource for information about the toxicity of household products and treatment of accidental exposures is the National Capital Poison Center at Georgetown University, 625-3333. Follow-Up: Mosquitoes

About twenty readers wrote to tell me their non-medical remedies to keep mosquitoes from biting. The winner, hands down, was Avon's Skin-So-Soft Bath Oil. Testimonies came from far and wide -- Gaithersburg, Arlington, Silver Spring, the District, Annandale and Dunkirk (Maryland, that is).

S-S-S, as some readers affectionately referred to it, had been field-tested in such mosquito-infested places as Alaska (in the summer), Hilton Head and the Bahamas, and while camping, fishing and sailing. Readers recommended diluting it with water before applying, perhaps half and half, and endorsed its use for men. Why it should work, I don't know. S-S-S is mainly mineral oil with fragrance and dispersing agents.

Two readers suggested taking vitamin B-1 or vitamin B complex before going outside during insect prime time. One reader advocated dabbing on citronella oil, which is available in drug stores and is used to make mosquito repellent candles. (Caution: citronella oil may cause a rash in people with sensitive skin.)

While I can't personally confirm the effectiveness of any of these products, they certainly seem harmless. For those readers who wrote asking me to send them the list of "natural" insect repellents, here it is. Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center in Northeast Washington. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Questions cannot be answered individually.