Dr. Joseph Mercola sound like a bit of a quack.
FDA Warns Mercola for Selling Bogus COVID Treatments
— Controversial physician in trouble for claims about vitamin C, vitamin D, and quercetin products
by Kristina Fiore, Director of Enterprise & Investigative Reporting, MedPage Today March 5, 2021
https://www.medpagetoday.com/special-reports/exclusives/91487
This was a research letter, not a study. It might be retracted as the author is being outed as having an anti-mask agenda and his measuring device is being attacked for not having the sensitivity to measure the levels he is claiming.
(JAMA Pediatrics. 2021;published online June 30)
Criticism in the comments section noted that the authors use an CO2 incubator analyzer (0-20%) that has an accuracy of 1% of the range (i.e. 2000ppm) and a time response T^90<=20 seconds for CO2. This device is unsuitable for measuring the transient concentrations during the respiration processes.