Both Innerspring and foam mattresses get heavier very year.
The idea behind the claim is true. Mattresses do gain weight over time as they absorb dead skin , colonies of dust mites (which feed on dead skin), oil and moisture.
When you flip your mattress over every six months to prevent it from getting worn in certain often used places it may come to your attention that it seems that your mattress gets heavier each time. This isn't your imagination. Your mattress is gaining weight!
Internet rumour mills claim that mattresses double in weight every 8 years. Probably not double but easily by several kilograms. It's well known that beds, in general, are a prime habitat for dust mites where one third of their nasty and brutish life occurs.
According to materials published by Ohio State University, a typical used mattress may have 100,000 to 10 million mites inside. Ten percent of the weight of a two-year-old pillow can be composed of dead mites and their droppings. Mites prefer warm, moist surroundings such as the inside of a mattress when someone is on it. One of their favorite foods is dead skin, and people shed about one fifth of an ounce of the stuff every week, some of which surely ends up flaking into your mattress. (Also gross: About 80 percent of the material seen floating in a sunbeam is, in fact, flakes of dead skin. Dust in the home is dead skin mostly.)
So with all those mites and dead skin, the mattress is bound to be heavier than a new one but doubling in weight is probably exaggerated.
Most mattresses have been constructed out of cotton. In theory this is a good thing. Cotton is fairly economical and very hard wearing, which is why most of our clothing is made out of it. The problem with organic fibers like cotton is that they attract mites and skin cells. With clothing, this isn’t a big deal since they are regularly washed.
The problem with all this stuff is that it doesn't just weigh your mattress down, it can also lead to some mattress health problems. Each time your body shifts on the mattress, you release a small cloud of the debris that are caught in your mattress's fibers, which you then breathe in. Constant exposure to all this mattress pollution can create respiratory problems including asthma, allergies, and sinus problems.
The best way to make sure you continue to have a clean space to sleep at night that doesn't give you mattress health related nightmares is to replace your old mattress as soon as you notice your current one gaining weight significantly. More people are getting allergies these days due to pollution, over use of antibiotics and medicine, processed and GM foods and over sanitization during childhood. One fifth of chronic diseases are allergies related.
Mattresses need to be changed in 3 to 5 years for a heathy sleep.