Property: Doctor on the House - Beware a chemical generation of spaced-out rats

The test for the safety of timber treatment chemicals is known as the LD50. This may sound like something you spray on your bike chain, but it stands for Lethal Dose 50 per cent; that is, the amount of the chemical that will kill half the population.

Chemicals affect different species in different ways, so the tests to find safe limits for people are carried out on other mammals, mainly rats. The LD50 is expressed in milligrams per kilogram of bodyweight (mg/kg), so if, for example, a 1mg dose is found to kill half the population of a group of rats with average body weight of 200g, then the LD50 would be 5mg/kg. This would put it in the deadly poison category, along with a substance like arsenic pentoxide (LD50 - 8mg/kg), which is commonly used to pre-treat building timbers.

If you use toxic chemicals in places where people come into contact with them, such as their homes, then there has to be some kind of testing, and measuring how much it takes to kill a fellow creature is one method. But what, I always wonder, about the other 50 per cent of the rats, the ones that didn't die?

I mean, are they still happily running around in their little wheels, or are they starting to feel a bit off their food? And what about their mental state? You couldn't blame them for feeling a bit depressed, what with half their mates dropping dead. But can you tell whether their near- lethal dose has left them confused, nauseous, weak or short of breath - the symptoms often reported by people who claim to have suffered pesticide poisoning? Not unless you speak rat and interview them.

So, understandably, there has not been much published about non-fatal exposure to timber treatments, and this has been a problem for the hundreds of people who have been affected by chemicals sprayed in their homes. The LD50 is only a study of acute toxicity, not of the chronic effects of exposure to lower levels over a longer term. And remember, the timber treatment companies give 25-year guarantees, so they must reckon on the stuff staying active for that length of time.

So, people have had timber treatment chemicals sprayed in their homes and then felt ill, but their symptoms have been quite general - headaches, runny nose and so on - and they may not have associated them with the treatment. If they have, then they will probably have felt better in a few days, and put it down to "just one of those things". If they felt like complaining, they probably wouldn't know who to complain to. For this reason it is thought that cases of pesticide poisoning are very largely under-reported. The chemical companies say that pesticide poisoning is very rare, because so few cases come to their attention.

Now, I don't know what the long-term effects of living in a house sprayed with pesticides are; nobody knows. But I do keep thinking about that LD50 test; because it means that a baby weighing, say, 4kg, will be affected by a dose of chemical 20 times more than an 80kg adult. And babies and children do tend to crawl around on the floorboards and put things in their mouths. I really don't want to scare anyone; I just think it is time we did some proper research and found out the facts.

q You can contact Jeff Howell at the Independent on Sunday or by e-mail on: Jeff@doctoronthehouse. demon.co.uk