This question is the subject of much debate and currently cannot be answered with any degree of finality.  Hence, in the hope of stimulating further research on the subject, we briefly outline several possible ways, both direct and indirect, by which elevated levels of atmospheric CO2 may enhance and/or diminish the physical well-being of the human race.

With respect to the direct health effects of CO2-enriched air, it is known that very high concentrations of atmospheric CO2 can produce a state of hypercapnia (Nahas et al., 1968; Brackett et al., 1969; van Ypersele de Strihou, 1974) or an excessive amount of CO2 in the blood that typically results in acidosis (Poyart and Nahas, 1968; Turino et al., 1974), which is a serious and sometimes fatal condition characterized in humans by headache, nausea and visual disturbances.  However, several studies have indicated that these phenomena have little to no negative impact on human health until the CO2 concentration of the air reaches approximately 15,000 ppm (Luft et al., 1974; Schaefer, 1982), which is 40 times greater than the air's current CO2 concentration and far higher than any concentration that could ever be produced by the burning of fossil fuels.

It would thus appear we have little reason to be concerned about the acute adverse health effects that are known to be associated with inhaling highly-CO2-enriched air.  But, we must ask ourselves, could there possibly be some as-yet-undiscovered longer-term effects, which might even prove to be positive, that are associated with breathing less-highly-CO2-enriched air, of perhaps double, triple or quadruple the current atmospheric CO2 concentration?

This question may seem strange to some, but it has good reason to be asked; for a review of the topic by Idso (1989) identified a number of CO2-sensitive physiological processes in humans that could well be altered in a CO2-enriched world.  Unfortunately, we have not the space to discuss this topic in any detail here; but a number of papers that provide a good starting point for delving deeper into the subject may be found in the volume edited by Nahas and Schaefer (1974), while others are cited by Mitz (1979).

It must be acknowledged, however, that the potential direct effects of continuously inhaling modestly-CO2-enriched air are highly speculative; but that fact only intensifies the need we have to study these phenomena in greater detail.  If there is reason to believe they may play a significant role in human health - and there is - it is imperative that we gain a better understanding of them; for who can say but what they might ultimately be found to have the capacity to totally overpower the potentially deleterious indirect health effects of atmospheric CO2 enrichment.

Turning our attention to these latter phenomena, we encounter the health effects of increasing temperature ... if one believes in CO2-induced global warming.  And if one does, one will find that increases in air temperature are, in the mean, beneficial to human health, as they eliminate far more cold-induced health problems than they create heat-induced health problems, as may be readily verified by perusing the many materials filed under the several subheadings of Health Effects (Temperature) in our Subject Index.

We come finally, then, to the indirect health effect of atmospheric CO2 enrichment that prompted our Editorials of 30 October 2002 and 20 November 2002, i.e., the potential for CO2-induced alterations in the compositions of the tissues of agricultural and medicinal plants.  And the question we ask ourselves - plus the many scientists who should be putting forth much greater effort to answer it - is this: Does atmospheric CO2 enrichment lead to changes in plant composition that will enhance or diminish human health?

We begin by noting, as indicated by Loladze (2002), that CO2-induced increases in plant productivity sometimes lead to reductions in essential element (E) to carbon (C) ratios in the edible tissues of various crops; and we affirm that it is not unreasonable to presume that these E/C reductions - where E represents an essential element such as nitrogen, iron, iodine or zinc - could intensify some of the serious health problems associated with human malnutrition.  However, this scenario may not be as bleak as it appears; for the E/C ratios of some essential elements in certain crops will likely not be reduced as a consequence of atmospheric CO2 enrichment; and in many of the cases where E/C ratios do decline, there are simple ways of adequately compensating for their reductions.

Consider, for example, the essential element nitrogen (N), which plays a key role in the production of proteins.  Loladze (2002) cites a CO2-enrichment study in which the concentration of N in rice grains dropped by an average of 14%; and if this result could be shown to be typical of rice, it would indeed be disconcerting, as rice is the primary food of over 50% of the world's population (Wittwer, 1995).  However, in the recent meta-analysis of Jablonski et al. (2002), it was found that when the results of two other CO2-enrichment studies of rice were combined with the one referenced by Loladze, there was, in the mean, no reduction in grain N concentration.

Of course, there is always the possibility that a larger number of studies could end up throwing the mean change in grain N concentration back into negative territory; but if it did, there is a simple solution to the problem.  With a concomitant mean increase of fully 38% in the mass of rice grains produced in the high-CO2 air of the 58 different studies analyzed by Jablonski et al., all that the people who depend upon rice for their sustenance would need to do to compensate for the small N deficiency would be to eat a little more of the much-more-plentiful grain.

And we must not forget the other possibility: more studies just might indicate a mean increase in the N concentration of rice grains!  Hence, whatever the outcome, based on the data currently available, it would seem that the ongoing rise in the air's CO2 content would likely not contribute to the development of a significant protein deficiency in the more than 50% of the population of the planet that owe their lives to the most important of all the world's crops, i.e., rice.

Moving on to some other crops, Jablonski et al. additionally found no mean CO2-induced decrease in seed N concentration in the legumes they investigated, based on the results of four separate studies.  This finding is also encouraging, since legumes provide fully 20% of the protein ingested by humans throughout the world (Wittwer, 1995).  What is more, the biomass of the CO2-enriched legumes, like that of rice, was found by Jablonski et al. to again be significantly increased, and by a whopping 56%, based on the results of 48 different studies.  Hence, in the case of earth's important nitrogen-fixing crops, there will likely be a vast increase in the total amount of protein-producing nitrogen that can be made available to humanity in a CO2-enriched world.

In the case of wheat, however, Jablonski et al. did find a CO2-induced decrease in grain N concentration; but it has fortunately been demonstrated by Rogers et al. (1996), Pleijel et al. (1999) and Kimball et al. (2001) that higher levels of nitrogen fertilization have the capacity to totally thwart this CO2-induced decrease in wheat grain N concentration, as discussed by Idso and Idso (2001).  And this observation suggests that fertilizing nutrient-deficient soils with other essential elements might possibly do the same for them, i.e., prevent reductions in their concentrations in edible crop tissues.

In some cases, however, such fertilization may be difficult, if not impossible, to achieve.  So what happens then?  Very possibly, there could be some CO2-induced E/C reductions in agricultural produce; but even in these instances there are mitigating circumstances that may avert a serious negative outcome.  Situations where adequate fertilization is difficult to achieve, for example, usually occur in the world's poorer countries, where people are unable to acquire and properly apply needed soil amendments; and in these economically depressed circumstances, people are often faced with limited opportunities for calorie intake in addition to nutrient intake.  Blessed with the increased agricultural productivity made possible by elevated levels of atmospheric CO2, however, populations indigenous to such areas would probably significantly increase their intake of locally grown food to alleviate their calorie shortages and thereby automatically compensate for the slight reductions in crop E/C ratios that may be caused by the elevated atmospheric CO2 concentration.

But what if something really unusual happens, so in a high-CO2 world of the future some people truly do acquire smaller amounts of certain essential elements than they do currently?  If the truth be told, we really do not know what the ultimate consequences would be; for the many potential effects of elevated CO2 on human physiology that are described in the publications of Nahas and Schaefer (1974) and Mitz (1979) could possibly alleviate or compensate for the negative effects of the CO2-induced nutrient deficiencies.

It must once again be emphasized, however, that this latter scenario is highly speculative; and until more is learned about it, as well as several other aspects of the CO2-nutrition issue, we will not be able to assess this important aspect of the future very well.  And that is why we earnestly implore the host of CO2 researchers at work today to broaden their scope of inquiry and go after the data that are needed to test these several important ideas.

Literally thousands of laboratory and field experiments have demonstrated that elevated levels of atmospheric CO2 significantly increase agricultural productivity; but we know embarrassingly little about what they do to the health-promoting properties of that more plentiful produce or to the life-saving substances found in various medicinal plants.  There are, however, a few notable exceptions to this general observation; and they are perhaps worth mentioning, because of the tantalizing possibilities they portend.

In a long-term study of the effects of a 75% increase in the air's CO2 concentration on the growth and fruit production of sour orange trees, Idso et al. (2002) observed a similar increase in the production of fruit biomass, as well as increases of 2-15% in the vitamin C concentration of the juice of the much more plentiful fruit.  In the words of Idso and Idso (2001), "these findings take on great significance when it is realized that scurvy - which is brought on by low intake of vitamin C - may be resurgent in industrial countries, especially among children (Ramar et al., 1993; Gomez-Carrasco et al., 1994), and that subclinical scurvy symptoms are increasing among adults (Dickinson et al., 1994)."  Furthermore, as they continue, "Hampl et al. (1999) have found that 12 to 20% of 12-18-year-old school children in the United States 'drastically under-consume' foods that supply vitamin C; while Johnston et al. (1998) have determined that 12 to 16% of U.S. college students have marginal plasma concentrations of vitamin C."  Hence, Idso and Idso conclude that "since vitamin C intake correlates strongly with the consumption of citrus juice (Dennison et al., 1998), and since the only high-vitamin-C juice consumed in any quantity by children is orange juice (Hampl et al., 1999), the modest role played by the ongoing rise in the air's CO2 content in increasing the vitamin C concentration of orange juice could ultimately prove to be of considerable significance for public health in the United States and elsewhere."

With respect to medicinal plants, Stuhlfauth et al. (1987) found that a near-tripling of the air's CO2 content increased the dry mass productivity of the wooly foxglove (Digitalis lanata) - which produces the cardiac glycoside digoxin that is used in the treatment of cardiac insufficiency - by 63% under well-watered conditions and by 83% under water-stressed conditions, while simultaneously increasing the concentration of digoxin within the plant dry mass by 11% under well-watered conditions and by 14% under conditions of water stress.  Similarly, in a subsequent whole-season field experiment, Stuhlfauth and Fock (1990) found that a near-tripling of the air's CO2 concentration elicited a 75% increase in the dry weight production of D. lanata per unit land area and a 15% increase in the dry mass concentration of digoxin.

In a similar vein, Idso et al. (2000) found that a 75% increase in the air's CO2 content produced a 56% increase in the belowground bulb biomass of the tropical spider lily (Hymenocallis littoralis), while it significantly increased (by 6-28%) the bulb concentrations of several medicinal constituents that have been shown to be effective against lymphocytic leukemia and ovary sarcoma (Pettit et al., 1986).  These substances have also been proven to be effective against the U.S. National Cancer Institute's panel of 60 human cancer cell lines, demonstrating greatest effectiveness against melanoma, brain, colon, lung and renal cancers (Pettit et al., 1993), while exhibiting strong anti-viral activity against Japanese encephalitis and yellow, dengue, Punta Tora and Rift Valley fevers (Gabrielsen et al., 1992a,b).

These several observations would seem to us to warrant the launching of a major new research initiative dedicated to resolving the questions that we and Loladze have raised about CO2 effects on human health.  What we currently know is insufficient to provide the answers we require.  Indeed, we have barely scratched the surface of the wealth of information that may be readily obtained from well-designed CO2-enrichment studies.

Surely, we have a moral imperative to engage in this endeavor; for if we fail to do so, governments at all levels around the world could ignorantly embark upon policy paths that would lead their constituents in a direction they may one day wish they had never taken.  Conversely, our efforts could provide a much stronger rationale for governments to proceed as they may currently be inclined to do.  In either eventuality, if the necessary research is conducted before final decisions are made, what will be done will be done with eyes wide open, as opposed to the state of partial blindness in which we currently operate with respect to this most important subject.

View our current Sub-Editorial, Researchers Must Pay More Attention to The Role of CO2 in Human Nutrition and Health, written in collaboration with Loladze.

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