Dr Larry Goodyer replies: Of course, no substance, whether synthetic or natural, is completely "safe", in that there will always be people who react badly to products applied to the skin, be they cosmetics or insect repellents.
My point is that considering the widespread use over the past 40 years - it is estimated that 200 million people worldwide apply DEET at least once a year - there have been remarkably few problems. The biggest concern has perhaps been the small number of reports of adverse effects on the central nervous system in children, which for the most part cannot definitely be linked to the use of DEET.
Sensible use of DEET products, following the manufacturers' recommendations, would minimise any potential problems. Just because a substance is derived from plants, it does not necessarily mean that it is any safer than a synthetic one. The penny-royal that you mention is in fact five times more toxic than DEET.
There is also no good evidence that either Permethrin or DEET is linked to Gulf war syndrome. Indeed, Permethrin was not widely used in the Gulf war and organophosphate insecticides such as DDT are a more likely candidate.
If you are visiting areas of endemic malaria it is essential that your mosquito nets be treated with an insecticide such as Permethrin. In the UK no insecticide product can be marketed without the approval of the Health and Safety Executive (HSE), which carefully examines issues such as efficacy, safety and the likely effect on the environment. Any preparation for the treatment of clothing would also carry such HSE approval.
Dr Larry Goodyer is a lecturer in clinical pharmacy at King's College, London. Contact the Nomad Travel Health Helpline (tel: 0891 633414; calls cost 50p per minute).
Looking for action? Try Christmas in China
I am single and would like to go on holiday at Christmas and have some company at the same time.
The travel editor replies: Several companys offer holidays for single holidaymakers. Solo's (tel: 0181-951 2800) divides its groups into those aged 28-55 and those aged 45 to 69. It still offers a choice of about 20 holidays over Christmas and the New Year. Most exotic is a "Discover China" tour, with visits to Peking, the Great Wall, Xian, Shanghai and Guilin. The holiday costs pounds 1,585, including single rooms in first-class hotels, for 13 days half-board, departing on 22 December.
Or there is action galore on its Lapland week with cross-country skiing, a husky safari, and snowboarding. The cost is pounds 1,069 half-board in a twin room. On the nine-night half-board break at the Hotel Atalaya Park in Spain's Costa del Sol, you will have nine tennis courts to choose from, water sports, mountain bikes and lessons in ballet, yoga, Spanish dancing and archery, and two 18-hole golf courses. For the 45 to 69 age-group, the cost is pounds 1,195.
Solo's organises seven houseparty-style holidays with lots of fun and frolic - the cost of three nights' half-board (with all meals on Christmas Day) is between pounds 299 and pounds 399. Cascade (tel: 01495 764472), another organisation catering for the unaccompanied is arranging Christmas houseparties in Gloucestershire - four nights with meals included costs between pounds 235 and pounds 355.
Walks Worldwide (tel: 01332 230883) is running a 15-day hiking trip leaving on 18 December in the sparsely populated mountains of Jebel Sahro in Eastern Morocco. The tour starts and finishes in Marrakesh. The all-inclusive cost is pounds 760.
Lakes and Mountains Holidays (tel: 01329 844 405) is running a Cafe Society Viennese Christmas with a gala Christmas Eve dinner at Schloss Wiekersdorf; a Christmas night Mozart concert in the Hofburg Palace and a music and wine evening with songs, Viennese dances and dinner on Boxing Day. The price is pounds 595.
A range of historic holidays are on offer from Neal's Yard Agency (tel: 07000 783 704), from Christmas and New Year retreats in Devon and the Malvern Hills to 21-day tours of southern India, accompanied by guides and a meditation teacher. The cost is pounds 2,495 all-inclusive.Reuse content