Tun Phorsacphear, managing director of Jamson Pest Management: 'Have you seen the rats here? They are huge!' Photograph: Scott Howes

Tun Phorsacphear kills pests for a living. The 28-year-old managing director of Jamson Pest Management and Julius Thiemann had a skin-crawling talk – complete with dead specimens. Phorsacphear explained the difficulties of controlling rats, roaches and mosquitoes, and how the improper use of pesticides is putting the nation’s health at risk.

What are the most dangerous pests you fight?

Mosquitoes cause most problems in Cambodia. I have a relative who works as a doctor in a hospital. He told me that dengue fever is no longer only affecting just children massively but also adults. The problem is mosquitoes are very hard to get under control. If you kill all mosquitoes with sprays in only one house in a community with 50 houses, they will return after three days because they take refuge in the other houses until the insecticide loses its effect. If you want to fight mosquitoes in a community effectively, you have to kill all of them in 80 per cent of all the houses. My mission is to kill all the mosquitoes in Cambodia – but only rich people can afford pest control services.

Do people try to get the pests under control themselves?

Yes, many buy chemicals from Thailand and Vietnam in the big markets like O’Russey but cannot read the instructions for use because it is neither in Khmer nor English. The vendors don’t know how to use the chemicals either, so people don’t use it properly. They are not wearing special protection like breathing masks and protective suits and put their health at serious risk.

What are the risks of improper use of these chemicals?

They have a dangerous step-by-step effect, especially for farmers in the countryside who live far from hospitals. First, the skin itches, and after a while it can come off. People cough for a month but don’t go to a doctor because they don’t know where the symptoms come from. After a month or so, they suddenly start to cough up blood and only then they see a doctor. The long-term cause of theses chemicals is often lung and skin cancer.

What do you do about rats?

Usually rats are scared of cats, but have you seen the rats here? They are huge! Cats are afraid of the rats. People are disgusted with rats and they try to poison them or kill them in traps. The problem is that dead rats begin to smell. The rats smell their dead friends and know what they have eaten so they won’t eat the poison. But you can buy poison that promises the dead rat won’t smell. But it doesn’t work so well. I have tested it.

How did you test that?

I test all chemicals in a warehouse before I use them for the first time. I buy the rats for testing in the market or my staff catch them in traps. To test if the rat wouldn’t smell after it had died, I fed it the chemical. After three days, it died, and after five days, it began to stink, so I will not tell my customers it won’t. I also test chemicals on insects like bed bugs (which are hard to kill because they travel from human to human), and cockroaches.

So does that mean the fried insects like crickets, spiders and cockroaches you can buy and eat in the markets are killed with chemicals?

No, that would be too expensive for the people who catch the insects. Insects for eating are usually caught in big blue traps near rice mills.

What other vermin cause damage or disease in Cambodia?

After mosquitoes and rodents like rats, termites are the third-biggest problem. They cause millions of dollars worth of damage in Cambodia every year. The problem is that lots of wooden interiors are status symbols to people in Vietnam, Thailand and Cambodia. Termites love wood, but you never see them because they have their nests in the walls and the floor. Houses can have up to four different termite colonies, and people don’t notice until it is too late. Especially in the countryside, people don’t fumigate their houses because it is too expensive, so sometimes their houses just collapse.