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This website for an air conditioner claims:

A built-in ionizer helps stimulate blood circulation, improve lung function and prevent respiratory passage illnesses.

Does research indicate that ionizers in air conditioners have the stated benefits?

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There are three health claims being made:

  1. Ionizers help with circulation.

  2. Ionizers can improve lung function.

  3. Ionizers can prevent respiratory passage illness.

Claim 1 might be true, but evidence is scarce.

Proof: I spent quite a while poking around on Google scholar. There are plenty of patents, and (surprisingly) company spec sheets that show up there when searching for terms related to this claim, but I can only find a single study that looks at the issue.

This paper finds (via a randomized controlled trial with n=13) that sitting in an ionized sauna yields a statistically significantly higher pulse rate than sitting in an ordinary one. The article looks kind of pseudo-sciency, but it is published in a real journal, and does look like it's at least taking basic precautions in its experimental design. If someone has a better source for this, I'm very interested to see it!

Claim 2 is false, at least for those with asthma. It may be true for those with dust allergies who do not suffer from asthma.

Proof: The possibility of using these systems to remove dust from the air has been studied since at least the 1960's. This 1975 study showed improvement in morning airflow (i.e. after sleeping in the house, hence being exposed to dust) for asthmatics. However, this 2010 metastudy finds, based on 6 randomized controlled trials:

Based on the evidence currently available from RCTs, a recommendation cannot be given for the use of room air ionizers to reduce symptoms in patients with chronic asthma.

That said, this 2012 study establishes that ionizers are effective at killing dust mite populations (which are a common cause of "dust" allergies), but only when they operate for a long period. Mites in the open lasted less than a day, but mites in a "simulated mattress" took more like months (these values are extrapolated by me from the reported LD50 exposure times). So, it's possible that if you are not asthmatic, but nonetheless allergic to dust mites, these systems could be effective. You'd need to leave them running for a very long time though.

One might suppose based on their ability to kill dust mites, that these systems would then be effective at controlling dust induced asthma as well (I certainly expected this). However, the meta study addresses this point as well:

Trials of current chemical and physical methods aimed at reducing exposure to house dust mite allergens failed to find an effect (relative risk 1.01, 95% CI 0.80–1.27), asthma symptom scores (standardized mean difference 0.04, 95% CI 0.15–0.07), or in medication usage (standardized mean difference 0.06, 95% CI 0.18–0.07.

Here is a second 2013 meta study that considers only this issue. Their findings are:

Despite numerous experimental and analytical differences across studies, the literature does not clearly support a beneficial role in exposure to negative air ions and respiratory function or asthmatic symptom alleviation. Further, collectively, the human experimental studies do not indicate a significant detrimental effect of exposure to positive air ions on respiratory measures. Exposure to negative or positive air ions does not appear to play an appreciable role in respiratory function.

Claim 3 is likely true, at least for some diseases. However, the evidence is scarce.

Proof: Again, it's very hard to find good source data on this one. I could find one study, but it did show a very significant reduction in infection rates for air treated with an ionizer. This 2009 PLoS medicine article performed an extensive study on changes in TB rates in guinea who were exposed directly to the exhaust air of a TB ward. They find that using ionizers halves the infection rate for the animals (though using UV light drops it to a third of the original instead). The presumed mechanism of action is the removal of TB cells from the air via attraction to the ionizer, so it stands to reason that is might work in humans as well. However, I cannot find any study showing that this is the case. I also cannot find any studies showing an impact on any other common respiratory infection (influenza, colds, etc.). Again, if anyone has a better source, I'm very interested to see it!

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