Mice exposed to third-hand smoke developed brain and...

"Third-hand smoke exposure can cripple your brain and liver, affecting your mannerisms, increasing your risk of neurodegenerative diseases, and ruining your metabolism," the Mail Online reports. But the study it reports on involved mice not people.

That said, the researchers made considerable efforts to replicate the effects that third-hand smoke exposure would have on humans, so the findings are still a cause of concern.

Third-hand smoke is defined as the toxins left on surfaces such as carpets and furniture when somebody smokes.

The researchers found that exposing mice to materials which had been around second-hand cigarette smoke such as carpets, upholstery and curtains, was bad for their health. Changes were seen within a month and got progressively worse.

The study highlighted the impact on the liver and brain and on increases in insulin resistance, which could potentially lead to type-2 diabetes.

The public are generally aware of the harms of first-hand and second-hand smoke, but the study authors are eager to educate the public on the dangers of third-hand smoke. That said, these findings would need to be verified by further trials in people.

Where did the story come from?

The study was carried out by researchers from the University of California, and was funded by grants from the Tobacco Research Disease Related Program (TRDRP).

The study was published in the peer-reviewed journal Clinical Science. It is available on an open access basis and is free to read online.

Generally the Mail Online's coverage of this study was balanced and accurate though its headline would have benefited from making clear the study involved mice not people. It is also unclear why they reported that third-hand smoke could affect your "mannerisms". Previous research found that third-hand smoke could make mice more hyperactive, but neither the previous research or this latest study looked specifically at changes in mannerisms.

What kind of research was this?

This was an animal study which wanted to determine whether there is a minimum time of exposure to third-hand smoke with regards to harmful effects on the health of mice.

In a previous study by the same researchers, it was found that exposure to third-hand smoke affected skeletal muscle and multiple organs including the liver and lungs, as well as the healing of wounds. The researchers wanted to follow this up by looking into whether the length of time of exposure had an impact.

Mouse studies such as this are useful for early stage research when exploring the cellular effects following exposure to different substances. However, although genetically there are many similarities between mice and humans, we aren't identical.

What did the research involve?

The researchers placed common household items such as curtain material, upholstery and carpet in empty mouse cages, and exposed them to second-hand smoke.

The mice were then kept in these cages with the materials that had been exposed to smoke. One, two, four or six months after exposure, tissues and serum were collected and analysed for changes in 24 biomarkers. The findings were compared to equivalent findings for mice kept in clean air.

The researchers reported that mice were exposed to levels of third-hand smoke that, in humans, would be equivalent to sharing a house with a smoker.

What were the basic results?

Overall, exposure to third-hand smoke over time was found to have numerous effects. Of particular interest is that the researchers were able to identify how soon after exposure the mice built up resistance to insulin. They found that just four months of exposure to third-hand smoke could result in the mice having increased susceptibility to developing type 2 diabetes.

Additionally, this research highlights the effect of third-hand smoke on the brain and liver, tissues in which changes were observed after just two months.

More specific changes in biomarkers:

After just one month of exposure to third-hand smoke, there were 1.5 to 2.5 fold increases in the following biomarkers:

aspartate aminotransferase (AST) – a liver enzyme that increases due to liver damage
granulocyte-macrophage colony stimulating factor (GM-CSF) and tumour necrosis factor (TNF-α) – both involved in inflammation and the immune response
adrenaline – a hormone associated with stress

After two months, the researchers found:

increased fasting blood glucose levels
increased Interleukin-6 (IL-6) – another marker of inflammation
hydrogen peroxide levels in the liver – indicating lower antioxidant ability of the liver
DNA damage in the liver

After four to six months of exposure, they found:

increased serum insulin levels – which could lead to type 2 diabetes
DNA damage in the brain and further damage in the liver
increased corticotropin-releasing hormone (CRH) levels – a hormone involved in the stress response

How did the researchers interpret the results?

The researchers concluded: "Time-dependent THS exposure has a significant effect on health as early as one month after initiation of exposure and these alterations progressively worsen with time. Our studies are important because virtually nothing is known about the effects of increased THS exposure time, they can serve to educate the public on the dangers of THS, and the biomarkers we identified can be used in the clinic, once verified in exposed humans."

Conclusion

This study investigated whether there is a minimum time of exposure to third-hand smoke with regards to harmful effects on the health of mice.

It found that exposure to third-hand smoke has several potential implications for health, with some changes observed as early as one month post exposure, and that the effects get progressively worse. It particularly highlighted the effects on the liver, brain and metabolic diseases such as type-2 diabetes following insulin resistance.

The public are generally aware of the harms of first-hand and second-hand smoke, but this study draws attention to the dangers of third-hand smoke.

But it's important to bear in mind that animal studies are very early stage research and these findings would need to be further investigated in people, perhaps through a cohort study. Additionally, this study only looked at exposure to third-hand smoke for up to six months, and it would be useful to determine the effects on health over a longer period.

Still, the results would suggest that exposure to tobacco smoke can be harmful even if it occurs on a "third-hand" basis.

If you are a smoker then you should try to never smoke indoors, especially if you have children living with you, though ideally you should quit smoking.