How Environmental Toxicants Contribute to Allergy, Asthma, and Autoimmunity

Walter J. Crinnion, ND

The rates of allergy, asthma, and autoimmunity have been increasing worldwide for the last few decades. Various theories have been proposed to account for this alarming trend. One of them is the effect of environmental toxicants. Epidemiological research has correlated exposure to multiple environmental chemicals (eg, pesticides, air pollutants, and solvents) with increasing rates of allergy, asthma, and autoimmunity. Studies performed in vitro and in vivo have documented the immune-altering effect of multiple common environmental chemicals. These common compounds lead to a diminished helper T cell type 1 (T_H1) and an enhanced helper T cell type 2 (T_H2) imbalance.

T_H1 and T_H2 Balance

Communication in the immune system occurs by chemical messengers that are typically referred to as cytokines or lymphokines. Included among these chemicals are interferon-gamma (IFN-gamma), a number of interleukins, tumor necrosis factor, and a host of other compounds. Helper T cells have begun to be categorized by the cytokine chemicals that they release as either T_H1 or T_H2. Helper T cells type 1, which produce primarily IFN-gamma and interleukin 2 (IL-2), command the cell-mediated immune functions of combating viral and bacterial pathogens, providing delayed-type hypersensitivity response, and fighting cancer cells (via natural killer cells). Helper T cells type 2 produce higher levels of IL-4 and IL-5, which stimulate the functioning of the humoral immune system. These cytokines not only boost the functioning of either the cellular or humoral immune response but also inhibit their rivals. Interferon-gamma inhibits T_H2-associated functions, while IL-4 and IL-10 inhibit T_H1-associated functions.¹ Helper T cell type 1 dominance is thought to predispose to organ-specific or tissue-specific autoimmunity (eg, arthritis and diabetes mellitus). Helper T cell type 2 dominance appears to lead to increased rates of allergy and systemic autoimmunity (ie, systemic lupus erythematosus [SLE]),² both of which are commonly seen in persons with chemical overburden.

The classic picture of immunotoxicity is reduced cell-mediated immunity, leading to chronic infections (decreased T_H1 function), allergies and asthma (increased T_H2 function), and chemical sensitivity and autoimmunity (either overactive T_H1 or T_H2 function).³ The first manifestation of immunotoxicity often seen in a case history is the development of asthma and allergies. Multiple environmental toxicants are associated with increased incidences of allergic reactivity to the environment and to foods. The chemicals that lead the way in causing atopic reactions include both organophosphate and chlorinated pesticides, solvents, and combustion by-products, including tobacco smoke and diesel exhaust.

Allergies and Indoor Air Pollutants

Environmental tobacco smoke is often thought of as the biggest indoor air pollutant. Combustion by-products from vehicles and cigarettes have been shown to lead to higher rates of allergy and asthma.⁴ Exposure to cigarette smoke reduces IFN-gamma⁵ and natural killer cell⁶ activity, which are T_H1 functions. This decrease in T_H1 function has been linked to reduced ability to fight respiratory infections, as well as having a role in increased cancer risk. Children exposed to environmental tobacco smoke are also more likely to be plagued by recurrent ear infections than other children without that exposure.⁷ While tobacco smoke reduces T_H1 response, it also enhances T_H2 response through greater production of IL-4, IL-5, and other proinflammatory cytokines, leading to increases in allergenicity⁸ and predisposing to autoimmunity.

Perfluorocarbons

Perfluorooctane sulfonate is the key ingredient of stain-repellent and durable water-repellent products used in fabrics and carpeting. It was found ubiquitously among US residents (mean urinary level, 20.7 mcg/L) in the Centers for Disease Control and Prevention’s Fourth Report.⁹ All materials in the home that are guarded by stain and water repellents (eg, carpeting, upholstery, and clothing) will release these perfluorooctane sulfonates into the air of the home. Exposing mice to perfluorooctane sulfonate resulted in a reduction of IFN-gamma and IL-2 production, while IL-4 and IL-5 production was increased, tipping the immune system to a T_H2 dominance.¹⁰ Prenatal exposure to these fluorocarbons resulted in mice being born with higher IgE levels and increased allergic reactivity.¹¹

Plasticizers (Phthalates)

Phthalate compounds are added to plastics to make them more flexible. They are not strongly bound to the plastics and are therefore released into the surrounding environment quite readily. Similar to the stain-repellent and water-repellent compounds in the home, anything containing phthalates (shower curtains, raincoats, toys, polyvinyl chloride flooring, furniture polishes, plastic food wrap, personal care products, fragrances, etc) will release these chemicals into the environment, where they will be picked up by the dust and carried throughout the home.¹² Phthalates have been associated with higher rates of allergic and respiratory problems. The presence of benzylbutyl phthalate in house dust is associated with rhinitis and eczema, and di-2-ethylhexyl phthalate is associated with asthma.¹³ Di-2-ethylhexyl phthalate is in all vinyl chloride products, and benzylbutyl phthalate is in both vinyl tiles and carpet and in some artificial leather products. Phthalates have been shown to induce T_H2 immune responses, with increased T_H2 cytokine production and increases in IgE and IgG.¹⁴

Solvents

Because trichloroethylene (TCE) has contaminated so many groundwater sites, the National Exposure Registry has been tracking adverse health effects for persons exposed to TCE. When Wistar rats were given water containing small amounts of TCE, similar to what homeowners would get by drinking their tap water, they produced increased levels of the T_H2 cytokine IL-4 and developed type 1 allergic reactivity.¹⁵ The National Exposure Registry has been tracking over 4000 individuals living in homes supplied with TCE-contaminated well water, all being near Superfund sites; as the cumulative exposure level increased, persons exhibited significantly more problems with respiratory allergies, asthma, emphysema, stroke, and hearing impairment.¹⁶

Children exposed to indoor solvents (from cleaning supplies, building materials, paints, and environmental tobacco smoke) have an enhanced T_H2 immune response and are much more likely to become reactive to milk and egg whites.¹⁷ The solvents leading to this proallergic state include toluene, orthoxylene, metaxylene, and paraxylene (xylenes are found in cleaning compounds), as well as ethyltoluene, ethylbenzene, and chlorobenzene.

Allergies and Outdoor Air Pollutants

In 2010, diesel vehicles comprised 70% of all new vehicle sales in France and 50% in the rest of Europe.¹⁸ In the United States, the sale of diesel vehicles increased by almost 40% in 2010. While diesel engines used to be mainly found in large trucks, earth-moving equipment, ships, buses, locomotives, and power generators, they are now commonly found in passenger vehicles on every roadway in the world. Diesel exhaust particle (DEP) exposure leads to increased rates of allergic reactivity and asthma, along with elevated production of antigen-specific IgE and histamine. When mice were exposed to ovalbumin after DEP exposure, they produced more IgE than mice who were not exposed to DEP.¹⁹ Recognizing the increased use of diesel engines and the increase in atopic response with DEP exposure, it is quite possible that diesel exhaust alone could be a major player in the worldwide increase in asthma.

The mucosal immune response in humans to DEP exposure was observed to see if the increased allergic tendencies found in animals also occurred in humans. In volunteers exposed to DEP, the levels of messenger RNA (mRNA) for cytokine production (IL-4, IL-5, Ill-6, IL-10, and IL-13) were increased after DEP exposure, revealing an elevated T_H2 response.²⁰ The researchers also found elevated levels of IL-4 in the nasal lavage fluid after exposure. Exposure to DEP in anther group of human volunteers showed that diesel exhaust directly caused a reduction in IFN-gamma production after exposure.²¹ Other studies have confirmed the increase in T_H2 cytokine production after DEP exposure and have shown that this exposure leads to increased reactivity to ragweed,²² cedar,²³ or birch²⁴ pollen and even egg protein.²⁵ Simultaneous exposure to DEP and the allergen will lead to more rapid development of allergic reactivity to foods or airborne allergens than would be seen from allergen exposure without DEP.²⁶ Other researchers demonstrated in an in vitro study²⁷ that DEP leads to a clear suppression of IFN-gamma, thereby reducing T_H1 function, while not affecting IL-4 and IL-5; exposure to DEP also reduced T_H1 function by inhibiting IFN-gamma mRNA expression and protein production more potently than dexamethasone or cyclosporine A. This reduction of T_H1 function and cell-mediated immunity is the likely cause for DEP being associated with respiratory infections.^28-31

Autoimmunity From Toxicant Exposures

Twelve persons who were residentially exposed to the organophosphate pesticide chlorpyrifos were checked for immune function from 1 to 4½ years after the exposure occurred.³² These individuals showed far higher risk for developing autoantibodies to important tissues than the rest of the population. The most common autoimmune antibodies found were anti–smooth muscle, affecting primarily the liver (odds ratio, 3.99), and anti–parietal cell, leading to diminished hydrochloric acid production (odds ratio, 9.7), as well as antithyroid and antimyelin antibodies.

Four groups of patients with long-term formaldehyde exposure showed similar autoantibody production.³³ The 4 groups comprised mobile home dwellers, office workers in closed buildings, persons who had been removed from their formaldehyde exposure for 1 year, and individuals with occupational exposure. Mobile home dwellers had a risk for anti–parietal cell antibodies that was 144-fold higher than that of non–mobile home dwellers. Office workers were 40.5-fold more likely to have anti–parietal cell antibodies. Levels of anti–smooth muscle, antimitochondrial antibodies, and anti–brush border were also higher in those with formaldehyde exposure. A previous study³⁴ of 8 persons with chronic low-level formaldehyde exposure showed a reduction in cell-mediated immunity, as would be found with T_H1 reduction.

A study³⁵ of 298 persons working with chemicals in the computer manufacturing industry showed more problems with reduced cellular immunity and increased autoimmune markers compared with control subjects. The most commonly found autoantibody was against myelin basic protein, which correlated with their self-reporting of headaches, mental status changes, and peripheral neuropathies.

Systemic lupus erythematosus has long been associated with numerous pharmaceutical compounds,³⁶ so its association with environmental chemicals should not be a surprise. The rates of SLE in the US population vary from 14.6 to 50.8 cases per 100 000.³⁷ African Americans generally show higher prevalences of 53 cases per 100 000 men and 168 cases per 100 000 women; however, in the neighborhood of Newton, in Gainesville, Georgia, where homes are close to local industry, the rates of SLE are far higher.³⁸ The SLE rate among this African American community is 1000 cases per 100 000 persons, a 6-fold elevation over national statistics. Other chemicals can lead to higher SLE rates. Regular, repeated exposures to hair dye have also been associated with increased rates of SLE.^39,40 Persons exposed to TCE contamination in city drinking water also had higher rates of SLE,⁴¹ as do persons who smoke. Being an active smoker increased the risk for developing SLE to 6.69; the risk of developing SLE was even elevated for those who quit, with a 3.62 risk.⁴²

Conclusion

Multiple commonly encountered xenobiotic toxicants have been clearly associated with an increased risk of allergy, asthma, and autoimmunity. The underlying mechanism for these immunological problems appears to be directly related to a T_H1-T_H2 imbalance that is a direct result of toxicant presence. These imbalances lead to decreased cell-mediated immunity and natural killer cell function, as well as increased allergic reactivity and systemic autoimmunity. The toxicants responsible for these imbalances are found in our everyday air and food, and those living in an urban area, close to roadways with high traffic and high diesel exhaust, are most at risk. Educating our patients about these common risks and providing options to help them reduce their exposures will accomplish both prevention and “physician as teacher,” aiding our patients in avoiding chronic health problems.

Walter J. Crinnion, ND has been in practice since 1982 with a special focus on treating chronic diseases that are caused by environmental toxic burden. He conducts post-graduate seminars in environmental medicine and is currently a professor at the Southwest College of Naturopathic Medicine where he is also the chair of Environmental Medicine. He is the author of Clean, Green and Lean (Wiley, pub.).

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