A mat of mineral yarns bonded with a binder, said yarns comprising a multitude of mineral filaments, and said yarn being cut to a length up to 100 mm characterized in that it includes an agent capable of trapping formaldehyde chosen from compounds comprising active methylene(s) corresponding to one of the following formulae to: FORMULA in which: - R 1 and R 2, which are identical or different, represent a hydrogen atom, a C 1 -C 20, preferably C 1 -C 6, alkyl radical, an amino radical or a radical of formula in which R 4 represents a radical or radical, where R 5 = H or -CH 3, and p is an integer varying from 1 to 6, - R 3 represents a hydrogen atom, a C 1 -C 10 alkyl radical, a phenyl radical or a halogen atom, - a is equal to 0 or 1, - b is equal to 0 or 1, - n is equal to 1 or 2, FORMULA R 6 -CHR 7 -C≡N in which: - R 6 represents a cyano radical or a radical in which: - R 8 represents a hydrogen atom, a C 1 -C 20, preferably C 1 -C 6, alkyl radical or an amino radical - c is equal to 0 or 1 - R 7 represents a hydrogen atom, a C 1 -C 10 alkyl radical, a phenyl radical or a halogen atom, FORMULA in which: - R 9 represents a -C=N or -CO-CH 3 radical - q is an integer varying from 1 to 4, FORMULA in which: - A represents a -(CH 2) 3 - or -C(CH 3) 2 - radical - r is equal to 0 or 1. FORMULA in which: - R 1 and R 2, which are identical or different, represent a hydrogen atom, a C 1 -C 20, preferably C 1 -C 6, alkyl radical, an amino radical or a radical of formula in which R 4 represents a radical or radical, where R 5 = H or -CH 3, and p is an integer varying from 1 to 6, - R 3 represents a hydrogen atom, a C 1 -C 10 alkyl radical, a phenyl radical or a halogen atom, - a is equal to 0 or 1, - b is equal to 0 or 1, - n is equal to 1 or 2, - R 1 and R 2, which are identical or different, represent a hydrogen atom, a C 1 -C 20, preferably C 1 -C 6, alkyl radical, an amino radical or a radical of formula in which R 4 represents a radical or radical, where R 5 = H or -CH 3, and p is an integer varying from 1 to 6, - R 3 represents a hydrogen atom, a C 1 -C 10 alkyl radical, a phenyl radical or a halogen atom, - a is equal to 0 or 1, - b is equal to 0 or 1, - n is equal to 1 or 2, FORMULA R 6 -CHR 7 -C≡N in which: - R 6 represents a cyano radical or a radical in which: - R 8 represents a hydrogen atom, a C 1 -C 20, preferably C 1 -C 6, alkyl radical or an amino radical - c is equal to 0 or 1 - R 7 represents a hydrogen atom, a C 1 -C 10 alkyl radical, a phenyl radical or a halogen atom, - R 6 represents a cyano radical or a radical in which: - R 8 represents a hydrogen atom, a C 1 -C 20, preferably C 1 -C 6, alkyl radical or an amino radical - c is equal to 0 or 1 - R 7 represents a hydrogen atom, a C 1 -C 10 alkyl radical, a phenyl radical or a halogen atom, - R 8 represents a hydrogen atom, a C 1 -C 20, preferably C 1 -C 6, alkyl radical or an amino radical - c is equal to 0 or 1 - R 7 represents a hydrogen atom, a C 1 -C 10 alkyl radical, a phenyl radical or a halogen atom, FORMULA in which: - R 9 represents a -C=N or -CO-CH 3 radical - q is an integer varying from 1 to 4, - R 9 represents a -C=N or -CO-CH 3 radical - q is an integer varying from 1 to 4, FORMULA in which: - A represents a -(CH 2) 3 - or -C(CH 3) 2 - radical - r is equal to 0 or 1. - A represents a -(CH 2) 3 - or -C(CH 3) 2 - radical - r is equal to 0 or 1.