Q: We like to go camping in the woods, but sure don't like the mosquitos. What's the best insect repellent to use?

A: Most insect repellents work fine, but they can be toxic.

If by best you mean most effective, that's a chemical known as DEET (N,N-diethyl-m-toluamide, or diethyltoluamide). It works against mosquitoes, biting flies, chiggers, fleas, gnats and ticks. No insect repellent works against stinging insects.

DEET gets absorbed through the skin and can cause allergic and toxic reactions. Local toxic reactions include skin burning, reddening and blistering -- even to the point of scarring. Rarely, internal toxic reactions have occurred in children, who have absorbed enough DEET to develop a serious form of brain poisoning known as toxic encephalopathy.

The most common local allergic reaction is hives. Internal allergic reactions, though rare, have included shock in one woman and a seizure apparently due to DEET in an 8-year-old girl.

My advice is to use insect repellents sparingly, and only when needed. Be especially careful when applying them to young children, who are more susceptible to bad reactions, and use a product with a low concentration of DEET.

Sprays generally have lower DEET concentrations than lotions, which run as high as 75 to 100 percent. Sprays don't last as long, but in many cases you may not need such prolonged protection. Off! spray is 15 percent DEET, and 6-12 Plus spray is 5 percent DEET together with 25 percent ethyl hexanediol, another insect repellent. Deep Woods Off! lotion contains 20 percent DEET, one of the lowest in lotions on the market.

If readers want to share any non-medical ways to keep mosquitoes from biting, write in and I'll put together a list in the next month or two.

Q: My doctor has recommended back surgery, namely spinal fusion and insertion of a metal rod to stabilize the spinal column. My problem is scoliosis, which has caused nerve compression and increasing back pain for four to five years. Where can I find information about scoliosis? Is there an age limit on surgery for this condition?

A: Scoliosis, from the Greek word for crooked, is an abnormal sideways curvature of the spine. Most cases are mild and require no treatment, but severe cases do. Because there are different types and causes, treatment varies. Although your symptoms sound severe, you might want to get a second opinion about the surgery and what you can expect from it.

Most cases of scoliosis are of unknown cause, with the rest being due to abnormal or malformed bones, or nerve and muscle disorders, such as polio or muscular dystrophy. Most of what I'll say about scoliosis will apply to the common type of unknown cause, referred to as idiopathic (literally, a disease peculiar to oneself, but now a term used to classify diseases whose cause is unknown).

Idiopathic scoliosis tends to run in families, and has several subtypes, depending on when it first appears -- infantile, before age 3; juvenile, age 3 to 10; and adolescent, from 10 to maturity, the type most people think of when they hear about scoliosis. In the adolescent type, girls outnumber boys about four to one.

It's estimated that about 4 percent of children age 10 to 14 have a slight curvature of their spine, although only half of these are serious enough to require medical follow-up. With so many children potentially affected, physicians and schools have started screening programs to identify scoliosis early.

The main complications from scoliosis are the cosmetic deformity, back pain and lung problems. To prevent or treat these, doctors use one of several methods -- back bracing and exercises, surgery or electrical stimulation.

Braces are used to correct the back deformity, and must be worn for long periods, typically 18 months full time and another 18 months half time. Surgery is used for more severe cases to align the backbone with metal rods or fuse the bones together. Electrical stimulation is a newer technique that triggers back muscles to contract in an attempt to straighten out the spine.

But the diagnosis and treatment of scoliosis isn't as simple as I've made it seem. There's been a lot of controversy recently about how serious a problem scoliosis is for most people who have it. For one thing, scoliosis seems to cause lung problems only in people with very severe curves. For another, back pain is usually not disabling. Because many affected individuals are treated, not much is known about what might happen if they weren't, and how much their curvature would otherwise increase.

There's even been some question about how effective current treatment is, because studies show that back curves sometimes return to their pre-treatment shape, regardless of the treatment used. Added to this is the potential risk of radiation from frequent back X-rays. All in all, these concerns have led some physicians to warn against the overdiagnosis, overtreatment and overexposure to X-rays of children with minor degrees of spinal curvature.

A conservative trend is developing in the way some physicians are treating scoliosis. This approach includes operating only on curves that are severe and progressing, or, in older individuals, curves that cause severe pain or lung problems. (Although most operations for scoliosis are done on adolescents, there's no age limit on surgery for people who are having serious problems.) Some experts recommend that screening programs likewise be conservative and avoid referring large numbers of adolescents who might be needlessly subjected to X-rays.

A good article discussing some of the current controversies about scoliosis is in the Dec. 6, 1985, issue of the Journal of the American Medical Association, available at most medical libraries. I don't think it requires a medical background to read. For more information about scoliosis, contact the local Scoliosis Association at 6280 Rathlin Dr., Springfield, Va. 22152; 569-5857.