Sperm defects caused by exposure to environmental toxins can be passed down the generations, research suggests.
Environmental toxins can damage sperm
Scientists say fathers who smoke and drink should be aware they are potentially not just damaging themselves, but also their heirs.
Tests on rats showed sperm damage caused by exposure to garden chemicals remained up to four generations later.
The US study was presented to the American Association for the Advancement of Science (AAAS).
It suggests that a father's health plays a greater role in the health of future generations than has been thought.
A team from the University of Idaho in Moscow tested the effects of a hormone-disrupting fungicide chemical called vinclozolin on embryonic rats.
The chemical altered genes in the sperm, including a number associated with human prostate cancer.
Rats exposed to it show signs of damage and overgrowth of the prostate, infertility and kidney problems.
The defects were also present in animals four generations on.
The scientists admitted that the rats were exposed to very high levels of vinclozolin.
Proof of principle
But they argued that their work shows that once toxins cause defects in sperm they can be passed down the generations.
Professor Cynthia Daniels, from Rutgers University in New Jersey, has written books on male and female reproduction.
She said men who drank a lot of alcohol had been shown to have increased rates of sperm defects; and nicotine from tobacco found its way into seminal fluid as well as blood.
Professor Daniels said: "We need to open up our eyes and look at the evidence.
"My advice to young couples would be moderation. Substances that have an impact on reproduction are often also carcinogenic.
"If I was a young man I would not drink very heavily and not smoke two packets of cigarettes a day while I was trying to conceive a child."
Professor Neil McClure, a fertility expert at Queen's University Belfast, UK, said the DNA in sperm cells was more tightly packed than in other cells, and so, to some extent, was protected from damage.
However, once sperm cell DNA was damaged, it had no mechanism by which to effect repairs.
He said: "There is no doubt that if you smoke like a chimney or drink vast amounts of alcohol it will result in sperm damage, and probably damage in the DNA of the sperm.
"My advice to any man trying for a baby would be to lead as healthy a lifestyle as possible."