A toddler scoops a cookie off the floor and shoves it in his mouth, lint, dog hair, ants and all.

It’s a familiar scene that makes some parents cringe and others shrug philosophically.

“No one has died yet,” said Susan Thomas, 40, mother of eight-year-old Helena. “If food drops on the floor I’m usually fine with kids or me eating it. Unless it has gotten visibly dirty or we recently got sprayed.”

Like many parents she believes in the five-second rule: food can’t be contaminated by dirt if it’s only on the ground for a short while.

Unfortunately for her, new research has put a damper on that theory.

A study by Aston University in the UK found that germs attack food as soon as it hits the floor. And it becomes more contaminated the longer it sits. In the span of 30 seconds, bacteria levels can increase by ten times.

This may be validation for germaphobic parents.

“I don’t have a five-second rule,” said stay-at-home mom Faryal Umer, 35. “My kids, Zoraiz and Izna [who are] seven and two, are strictly forbidden to pick up and eat anything from the floor.”

Crayons and Play-Doh were also banned from her house after her son tried to eat them.

Carla Greaves, 45, said: “I have always discouraged my seven-year-old daughter, Destiny, from eating off the floor. The five-second rule can work but it depends on how dirty the surroundings are and whether there are pets. Destiny likes to put things in her mouth — as a baby and even now — so we are constantly telling her to take things out.”

Diabetes educator Debbie Jones, 61, is the mother of three grown children. The nurse believes it’s never OK to eat off the floor.

“The food could have landed on a floor that had recently been sprayed with insecticide or germs from pets,” she said. “People walk on floors and there are germs on shoes and the soles of our feet. I would suggest that when food falls on the floor it needs to be put in the trash.”

Nutritionist Catherine Burns, 37, was on the fence. The mother of two girls, Chloe, seven, and Belle, five, said it depended on where the food fell.

“Some floors I would be OK with, at home for example, but others, such as on aeroplanes and at movie theatres, I wouldn’t.”

Verde Brown has a similar view.

“My daughter, Aiva is 16 months,” said the 37-year-old accountant. “We are fine with her eating off the floor because, to be honest, I know that the floor is clean. I wouldn’t want her to eat anything off the ground outside so much.”

A study by Clemson University in South Carolina seems to back that belief. It says the cleanliness of the floor is a more important consideration than how long the food is on it.

Food dropped on carpet might become a little fuzzier but is likely less contaminated than snacks eaten off tile. Bread or bologna dropped onto carpet treated with salmonella had less than one per cent of the bacteria transferred to the food, compared with 70 per cent on tile or wood.

Some scientists believe that the immune system is like a muscle and has to be worked to be strengthened.

Both Mrs Burns and Mrs Brown agreed.

Said Mrs Brown: “My 16-month-old has eaten dirt while I was gardening. Her sitter believes in playing outside and getting dirty. I have found her eating a leaf when picking her up.

“My husband is more OK with this than I am, so I’m reluctantly fine with it. To be honest I think it has strengthened her immune system. I am sick with a cold and the flu right now and she’s perfectly fine.”

Some scientists say that by making the world around our children too clean, we may be making their immune systems flabby. Children exposed to more animal faeces had more cases of diarrhoea before age two, but less incidence of inflammation as they grew into adulthood. Many chronic adult illnesses, such as heart disease and diabetes, are linked to inflammation.