Question: With all the attention these days about West Nile virus, how safe and effective are citronella candles in dealing with mosquitoes? - V.M., Orlando, Fla.

Answer: Citronella is actually an oil distilled from the leaves of an Asian grass that's a close cousin to the lemon grass used in Thai cuisine. The word "citronella" means "lemon-like". When citronella is used in the usual 3 percent concentration in candles or the 5 percent concentration in incense, the oil burns and gives off a smoke mosquitoes and other flying insects find rather noxious. It's not a poison to them, but rather an odor they'd just as well steer clear of. This time of year, you'll see lots of citronella candles and incense used outdoors to keep away the bugs. With West Nile virus and its risk of encephalitis, mosquitoes are more of a pest in the U.S. than ever before.

Does citronella actually work? Perhaps. First of all, there needs to be enough citronella smoke to keep them at bay. That means the more candles used, the better. The difficulty is that in open areas, especially if there's a breeze, it's tough to maintain a high enough level of citronella smoke to keep an open space free of mosquitoes. I suppose if you're right by the smoke you'll be less likely to get attacked. But in all likelihood, that's not going to happen. Research done in Ontario testing the efficacy of 3 percent citronella candles and 5 percent citronella incense found compared with non-treated subjects, those using the citronella candles had, on average, 6.2 bites in 5 minutes versus 10.8 bites in the control group; those using the incense had, on average, 8.2 bites. Those translate to a 42.3 percent and 24.2 percent reduction in bites, respectively. Still, they got more than a bite a minute. While that may be enough protection for some, I don't really want any bites.

What works the best to avoid getting attacked by mosquitoes? Since they grow and multiply in water collections, make sure there are no nearby puddles or bird water feeders to attract them. Bright-colored clothing attracts bugs, so wear neutral, light-colored clothing to avoid advertising your blood. The best insect repellents are the DEET-containing lotions or sprays. They're not recommended for use on infants and toddlers. 4 percent DEET sprays or lotions can be used sparingly on small children. The EPA states that DEET is safe for adults when used as directed.

Question: I was recently switched from the combo blood pressure pill atenolol/chlorthalidone to lisinopril/HCTZ because the beta blocker part of the pill (atenolol) was making me feel really washed out. I know both pills have a water pill, but what's the difference between HCTZ and chlorthalidone? - T.E., Philadelphia.

Answer: Both HCTZ (hydrochlorothiazide) and chlorthalidone are "thiazide" water pills that act in the kidney to block water reabsorption when blood is filtered. They're the two most commonly used diuretics for the treatment of high blood pressure. Unlike the common water pills Lasix and Bumex, their duration of effect is long enough to be useful for blood pressure control. They both can cause the excretion of potassium; HCTZ is often combined with the potassium-sparing water pill triamterene to offset potassium wasting and augment blood pressure lowering.

By itself, HCTZ lowers the systolic (upper) blood pressure reading by roughly 7 to 8 points and lowers the diastolic (bottom) blood pressure reading by roughly 5 points. In contrast, chlorthalidone is 1.5 to 2 times more potent and has about twice the duration of blood pressure-lowering effect. Also, there have been studies showing that chlorthalidone reduced heart attacks and strokes in folks with hypertension to a greater degree than HCTZ. Chlorthalidone does have a greater chance of dropping the blood potassium level because of its greater potency.

Why is HCTZ prescribed 20 times more often than chlorthalidone despite the potential superiority of the latter drug? The biggest reason is the lowest chlorthalidone pill available is a 25 mg unscored tablet. It often needs to be cut in half or quarters because it's too potent. HCTZ is often paired with the potassium-sparing diuretic, while chlorthalidone has no such combo. Besides atenolol/chlorthalidone, there's only one other chlorthalidone combo BP pill available: Edarbyclor.

Dr. Mitchell Hecht is a physician specializing in internal medicine. Send questions to him at: "Ask Dr. H," P.O. Box 767787, Atlanta, Ga. 30076.