Understanding the levels of evidence
Not all factors carry the same weight. Some are solidly documented, others are mere hypotheses, and a few are claims that studies do not confirm. The badge shown throughout the tool indicates where each factor stands.
A link with fertility is documented by numerous, converging human studies, endorsed by health authorities.
Example: Smoking: reduced ovarian reserve in women, impaired sperm quality in men.
Several human studies point the same way, with a plausible biological mechanism, but the evidence is not yet conclusive.
Example: Phthalates found in some scented cosmetics, associated with lower sperm quality.
Some signals exist (animal studies, small human studies) but results are conflicting and the scientific community is still debating them.
Example: Carrying a phone in a trouser pocket and sperm motility.
The exposure is real and common, but no serious study has measured its effect on human fertility. It remains unknown.
Example: Mixtures of food additives consumed together daily (cocktail effect).
A claim is circulating (social media, marketing) but available studies do not confirm it, and may even contradict it.
Example: Fluoride in toothpaste at normal doses presented as a cause of infertility.
A factor labeled "not studied" is not reassuring: it simply means no one has measured its effect. Conversely, an "established" factor does not cause infertility on its own.
See all factor sheets sorted by everyday context
Content last updated: September 14, 2026