About a century ago, allergy was so uncommon that it hadnt yet been named. The term hay fever had been around since the 1800s, but the word allergy wasnt coined until 1906.1

Three of the most common allergens: grass pollen (above right), ragweed pollen (above left) and dust mites (above far right).

Historically, allergy appears to have been a byproduct of the developed, industrialized world. Allergy in pre-war, pre-industrialized, 1930s Japan, for example, had not been recognized. But, by the 1980s, the country had advanced to a formidable industrialized nation, and allergy rates followed. By 1986, more than 30% of children in very polluted areas were identified with allergic rhinitis due to pollen.1

Similarly, in North America during the 1930s and 1940s, only about 3% of people (and up to 10% in ragweed-infested areas) were estimated to have hay fever (i.e., allergic rhinitis).1 During the past decades, however, the prevalence of allergic rhinitis has continued to increase, from 10% of the population in 1970 to 30% of adults and 40% of children in 2000, with severity ranging from mild to debilitating.2

The actual prevalence of allergic rhinitis is unknown because many of these allergy sufferers dont seek medical treatment. Currently, more than 14% of adults (one in seven) surveyed in the United States say theyve been medically diagnosed with nasal allergies; that equates to nearly 36 million Americans with allergic rhinitis.3,4

But, its not just allergic rhinitis. Many more Americans have other or additional allergies. At last count, more than half of all Americans (54%) test positive to at least one of the top 10 allergens.5 On average, people who test positive have at least three allergies.

Allergies are not only becoming more prevalent, but also more severe and more deadly. For instance, more than 12 million Americans have a food allergy, such as to seafood (6.9 million) or to peanuts or tree nuts (3.3 million), according to the Food Allergy & Anaphylaxis Network. Indeed, the number of children allergic to peanuts doubled between 1997 and 2002. In the United States, food allergy is the leading cause of anaphylaxis (outside of hospitals) and causes an estimated 150 to 200 deaths and 30,000 emergency room visits each year.

But, peanuts have been cultivated for thousands of years, and pollen has been around for millions of years. So, whats causing the sudden increasing severity and exploding prevalence of allergiesfrom almost non-existent a century ago to almost ubiquitous today?

 

Ocular Allergies on the Rise In the literature, ocular allergy has clearly been underestimated and underappreciated, says Leonard J. Bielory, M.D., co-director of the Immuno-ophthalmology Service at the University of Medicine and Dentistry of New Jersey. Dr. Bielorys research (soon to be published) shows that about 80% to 90% of people who have allergic rhinitis also have ocular allergic complaints. Thats nothing to sneeze at. Other research has shown that allergic conjunctivitis symptoms are at least as severe as rhinitis symptoms in approximately 70% of people with allergic rhinitis.1 Also, isolated ocular symptoms are more prevalent than isolated nasal symptoms during the summer months (May to August) in the United States.2 In a notable subset of allergy sufferers, their ocular allergy complaints are actually worse than their nasal allergy complaints, according to the Allergies in America survey.3 More than half of nasal allergy sufferers say that their red, itching eyes (53%), headache (51%) and watering eyes (51%) are extremely or moderately bothersome. Furthermore, red, itching eyes are the most bothersome symptom of 10% of nasal allergy sufferers, and watering eyes are the most bothersome symptom of 5% of nasal allergy sufferers. In terms of exposure, the eyes actually have it worse than the nose, says Paul J. Gomes, M.S., director of allergy at ORA Clinical Research and Development, in North Andover, Mass. Your eyes are open the whole day, so they are these two huge pollen-collecting dishesthe perfect entryway for allergens, he says. In fact, our studies have shown that if you put an allergen in the eye, not only do 80% of patients get eye allergies, but they also get nasal allergies. But, if you put an allergen up the nose, you only get nasal allergy; you dont get ocular allergy. Atmospheric pollution also damages the ocular surface, allowing allergens to penetrate the eyes defenses more easily, Mr. Gomes says. How it works: Air pollutants create low-level ozone, which results in oxidative stress on the conjunctival epithelium. This, in turn, potentially diminishes the protection provided by the tear film barrier. That one-two punch makes it much easier for allergens to penetrate into the eye, exacerbating the severity of allergy, and perhaps causing what would have been an occasionally mild complaint to become a more chronic, more prolonged intense reaction, he says. In short, Theres a huge unmet need in terms of the number of people with ocular allergy complaints, Mr. Gomes says. And, that need will only increase. In the future, ocular therapies will likely target the mast cell itself to preventnot just inhibitthe mast cell from degranulating. Fortunately, he says, current topical therapy with antihistamine/mast cell stabilizers can provide ocular relief for most patients most of the time.   1. Wthrich B, Brignoli R, Canevascini M, Gerber M. Epidemiological survey in hay fever patients: symptom prevalence and severity and influence on patient management. Schweiz Med Wochenschr 1998 Jan 31;128(5):139-43. 2. Singh K, Bielory L, et al. Epidemiology of ocular allergy symptoms in United States adults (19881994). Presented at the American College of Allergy, Asthma & Immunology Annual Meeting; November 9-15, 2006; Philadelphia. Abstract 34. 3. Allergies in America: A Landmark Survey of Nasal Allergy Sufferers: Executive Summary: Adult: 9. Available at: www.mmcpub.com/scsaia/AdultSummary.pdf. (Accessed July 21, 2008.) 4. Abelson MB, Lilyestrom L. Paving the pastures: Urban life and allergy. Rev Ophthalmol 2007 Feb;14(2):86-88.

Two Possible Causes

There are at least two basicseemingly opposingcauses: the dirty cause and the clean cause.

The dirty cause (or pollution hypothesis) suggests that increased pollution in our environment has overburdened and hypersensitized our immune systems.

The clean cause (or hygiene hypothesis) suggests that weve made our modern household environment so clean that our bodies overreact to otherwise harmless microbes and pathogens.

Lets take a closer look at each hypothesis.

 

The Pollution Hypothesis

Theres no question that increased pollution in our environment goes hand-in-hand with increased allergy. Indeed, a recent study of 3,000 children in Munich, Germany, confirms previous investigations that air pollution increases the risk for allergy and atopic diseases.6 Specifically, children who live close to high-traffic roads were 50% more likely to have allergies. And the closer they live, the higher the risk. Children living closer than 50 meters to a busy street had the highest probability of getting allergic symptoms, compared to children living further away, wrote author Joachim Heinrich, Ph.D.6

At the same time, rising carbon dioxide emissions (from cars, trucks and factories) are actually causing plants to boost pollen production. And, the worst is yet to come. Carbon dioxide in the atmosphere is expected to double by the end of this century. Such an increase would lead to an estimated 61% more pollen from ragweed.7

Pollution not only makes more pollen, but it also makes pollen more powerful as an allergen. In polluted areas, airborne particles accumulate on the surface of pollen grains and actually change the shape and structure of pollen. Also, pollution causes the release of more pollen protein on the surface of pollen grains. So, polluted pollen is more effective than nonpolluted pollen in inducing allergic reactions.8

Pollution also damages the bodys mucosal defenses, which allows greater access for allergens to invade. Specifically, pollutants aggravate the nasal mucosa, causing chronic inflammation and epithelial breakdown, and even altering the mucociliary system. This increases the opportunity of exposure to environmental allergens and jumpstarts the process of allergic sensitization.9

Indeed, such exposure has even led to a new diagnosis of an allergy-like conjunctivitis, urban eye allergy syndrome, that is not directly caused by allergy, but largely by pollution.10

 

The Hygiene Hypothesis

On the other hand, the hygiene hypothesis says that a cleaner environment, particularly in childhood, causes our bodies to overreact to everyday dirt. In other words, using antimicrobial soap might be causing your kid to sneeze.

Long before antimicrobial soap, purified water and dust-filtering air conditioning came along, humans (particularly children) were exposed to all kinds of dirt and microorganisms. (Of course, in poorer areas and underdeveloped countries, they still are.) The hygiene hypothesis holds that in dirtier environments, microbial exposure (to viruses, bacteria and parasites) effectively educates the bodys immune system, teaching it how to defend itself.

Take that education away (by creating a much cleaner environment), and it alters the immune systems development. Now, the body is more likely to mistake a relatively harmless allergen for an invasive pathogen, such as bacteria or a parasite. This, in turn, stimulates an overreactionan allergic reaction.

More specifically, an allergic reaction is an immunological hypersensitivity reaction that causes an immediate inflammatory response. In the immune system, T helper cells act as the traffic copsthey tell the immune system how to react to an invader. Bacteria and viruses generally elicit a TH1-mediated immune response. But when the immune system detects a greater danger, it fights back with a TH2 response (an allergic response).

The hygiene hypothesis suggests that insufficient stimulation of the TH1 arm of the immune system leads to an overactive TH2 arm, which in turn causes allergic disease.

The hygiene hypothesis has plenty of support. It started with a study that found that allergic rhinitis and eczema are less common in children from larger familieswho presumably expose each other to more infectious agentsthan in families with only one child.11 Likewise, growing up with a dog or a cat in the home appears to reduce the risk of developing allergies.12

Another classic example: Before the fall of the Berlin Wall in 1989, children in Eastern Germanywhere living conditions were poorer and dirtierhad lower prevalence of allergic rhinitis. Following unification, and the resulting surge in Western lifestyle, allergy rates in East German children began to catch up to their West German counterparts.13

Similarly, people who grow up in rural areas, particularly those who grow up on farms, tend to have lower risk for allergic rhinitis and asthma. Exposure to allergenic pollen in early life does not appear to increase the risk of acquiring symptoms of respiratory allergy, and may even give some protection against them, wrote the investigators of the International Study of Asthma and Allergies in Childhood (ISAAC).14

So, does the hygiene hypothesis mean than we can reduce allergies by reverting to a less hygienic lifestyle? Should we get rid of the antibacterial soap?

 

No Clear Answer

Unfortunately, neither the pollution hypothesis nor the hygiene hypothesis provides all the answers. Many questions remain.

Pollution, for instance, may not directly cause allergy. Instead, it may act as an adjuvantsomething that influences the course of an immune responseto exacerbate the reaction to an allergen.15

The hygiene hypothesis has some holes in it, too. For one, the inner-city poor, who are exposed to greater allergens and microbes (and would presumably benefit from the hygiene hypothesis), actually have higher rates of asthma and respiratory allergies.16

Also, according to the hygiene hypothesis, viral infections prevent the development of allergic disease. But, the influenza virus now appears to enhance, rather than inhibit, the development of asthma and allergic responses.17

Another curious anomaly: While the rates of allergic rhinitis and asthma have increased throughout the 20th century, they appear to have reached a peak among children in the early 1990sand may now be on a decline.18 The hygiene hypothesis doesnt explain such an anomaly, since concurrent environmental exposure certainly hasnt declined.

Also, allergies arent all environmental. Genetics play a role as well. For instance, if one parent suffers from allergies, the child is 40% more likely to have allergies; if both parents have allergies, their children are 75% more likely to have allergies, according to the American College of Allergy, Asthma & Immunology.

In other words, dont toss out that antibacterial soap just yet.

 

1. Jackson M. Allergy: The History of a Modern Malady. 1st ed. London: Reaktion Books; 2007:10,24,25.

2. Dykewicz MS, Fineman S, Skoner DP, et al. Diagnosis and management of rhinitis: complete guidelines of the Joint Task Force on Practice Parameters in Allergy, Asthma and Immunology. American Academy of Allergy, Asthma, and Immunology. Ann Allergy Asthma Immunol 1998 Nov;81(5 Pt 2):478-518.

3. Allergies in America: A Landmark Survey of Nasal Allergy Sufferers: Executive Summary: Adult:4. Available at: www.mmcpub.com/scsaia/AdultSummary.pdf. (Accessed July 21, 2008.)

4. Nathan RA, Meltzer EO, Selner JC, et al. Prevalence of allergic rhinitis in the United States. J Allergy Clin Immunol 1997;99:S808-S14.

5. Arbes SJ Jr, Gergen PJ, Elliott L, Zeldin DC. Prevalences of positive skin test responses to 10 common allergens in the US population: results from the third National Health and Nutrition Examination Survey. J Allergy Clin Immunol. 2005 Aug;116(2):377-83.

6. Morgenstern V, Zutavern A, Cyrys J, et al.; GINI Study Group; LISA Study Group. Atopic diseases, allergic sensitization, and exposure to traffic-related air pollution in children. Am J Respir Crit Care Med 2008 Jun 15;177(12):1331-7.

7. Wayne P, Foster S, Connolly J, et al. Production of allergenic pollen by ragweed (Ambrosia artemisiifolia L.) is increased in CO2-enriched atmospheres. Ann Allergy Asthma Immunol 2002 Mar;88(3):279-82.

8. Chehregani A, Majde A, Moin M, et al. Increasing allergy potency of Zinnia pollen grains in polluted areas. Ecotoxicol Environ Saf 2004 Jun;58(2):267-72.

9. Passali D, Lauriello M, Mezzedimi C, Bellussi L. Nasal allergy and atmospheric pollution. Int J Pediatr Otorhinolaryngol 1999 Oct 5;49 Suppl 1:S257-60.

10. Leonardi A, Lanier B. Urban eye allergy syndrome: a new clinical entity? Curr Med Res Opin 2008 Jun 28. [Epub ahead of print.]

11. Strachan DP. Hay fever, hygiene, and household size. BMJ 1989 Nov 18;299(6710):1259-60.

12. Ownby DR, Johnson CC, Peterson EL. Exposure to dogs and cats in the first year of life and risk of allergic sensitization at 6 to 7 years of age. JAMA 2002 Aug 28;288(8):963-72.

13. von Mutius E, Weiland SK, Fritzsch C, et al. Increasing prevalence of hay fever and atopy among children in Leipzig, East Germany. Lancet 1998 Mar 21;351(9106):862-6.

14. Burr ML, Emberlin JC, Treu R, et al; ISAAC Phase One Study Group. Pollen counts in relation to the prevalence of allergic rhinoconjunctivitis, asthma and atopic eczema in the International Study of Asthma and Allergies in Childhood (ISAAC). Clin Exp Allergy 2003 Dec;33(12):1675-80.

15. Takafuji S, Suzuki S, Muranaka M, Miyamoto T. Influence of environmental factors on IgE production. Ciba Found Symp 1989;147:188-201; discussion 201-4. Review.

16. Matricardi PM, Bouygue GR, Tripodi S. Inner-city asthma and the hygiene hypothesis. Ann Allergy Asthma Immunol 2002 Dec;89(6 Suppl 1):69-74. Review.

17. Umetsu DT. Flu strikes the hygiene hypothesis. Nat Med 2004 Mar;10(3):232-4.

18. Zllner IK, Weiland SK, Piechotowski I, et al. No increase in the prevalence of asthma, allergies, and atopic sensitisation among children in Germany: 1992-2001. Thorax 2005 Jul;60(7):545-8.