Respiratory

Lundberg and colleagues measured nitric oxide output from the nasal airways and found concentrations 15 times higher than in the lower airways or during oral breathing. Nasal NO is produced by epithelial cells in the paranasal sinuses and serves as a first-line antimicrobial, vasodilator and bronchodilator. Humming increases nasal NO output 15-fold above quiet exhalation, suggesting sinus ventilation dramatically amplifies the effect.

Nitric oxide at these concentrations is directly bactericidal and viricidal, providing an innate immune defence that mouth breathing bypasses entirely. It also enhances oxygen uptake in the alveoli by matching ventilation to perfusion. Nasal breathing is not a preference. It is a physiological system that produces a potent signalling molecule with immune, cardiovascular and respiratory functions.

The United Kingdom has 5.4 million people receiving treatment for asthma, including 1.1 million children. This represents one of the highest childhood asthma rates in the world. Prevalence has risen approximately 300% since the 1960s, a timescale too rapid for genetic change. The UK has three asthma deaths per day, the highest mortality rate in Europe.

The increase correlates temporally with indoor lifestyles, central heating, carpeted floors, reduced ventilation, processed food consumption, antibiotic use in early life and C-section deliveries. All of these reduce exposure to environmental microbes and alter the developing immune system toward allergic responses, consistent with the "old friends" hypothesis documented in the terminal's terrain domain.

Asthma prevalence in UK children trebled between the 1970s and the 1990s, rising from approximately five per cent to fifteen per cent. GP consultations for asthma reached their highest recorded levels in the mid-1990s. Anderson and colleagues, in a 2007 review of long-term trends, attributed the rise to a combination of factors: increased indoor allergen exposure from house dust mites, reduced microbial diversity in early life, rising indoor air pollutant concentrations and dietary changes reducing antioxidant intake.

The pharmaceutical market response was substantial. GlaxoSmithKline's Seretide (fluticasone/salmeterol) became the world's best-selling inhaled corticosteroid, generating over £5 billion in peak annual revenue. AstraZeneca's Symbicort reached comparable sales. Both products treat symptoms. Neither addresses the environmental and dietary contributors identified in the epidemiology. Between 2000 and 2020, UK prescriptions for asthma and COPD medications increased from forty two million to sixty eight million items annually.

Bowler and colleagues randomised 39 asthma patients to either Buteyko breathing retraining or a control group. After 12 weeks, the Buteyko group reduced bronchodilator use by 90% and inhaled corticosteroid use by 49%, with no deterioration in lung function or asthma control. McHugh replicated the bronchodilator reduction finding in a New Zealand trial.

The technique works by deliberately reducing breathing volume to restore CO2 tolerance, shifting the patient from chronic hyperventilation toward slower, nasal, diaphragmatic breathing. The Bohr effect predicts this would improve tissue oxygenation despite lower minute ventilation. Asthma UK acknowledged Buteyko as a complementary technique, though it remains absent from NICE guidelines and is not available on the NHS.

UK indoor environments contain 2 to 5 times higher concentrations of many pollutants than outdoor air. Volatile organic compounds from furnishings, cleaning products and cooking can exceed WHO guideline levels. CO2 concentrations in occupied UK bedrooms regularly reach 2,000 to 3,000 parts per million overnight, against an outdoor baseline of 420 ppm. Cognitive function measurably declines above 1,000 ppm.

Allen and colleagues demonstrated in a double-blind study that cognitive function scores dropped 21% at 1,000 ppm CO2 and 53% at 1,400 ppm compared to 550 ppm. The average UK adult spends 90% of their time indoors, breathing recirculated air with elevated CO2, reduced oxygen and accumulated chemical off-gassing. Modern energy-efficient buildings are sealed tighter than their predecessors, improving insulation at the cost of ventilation.

Harari et al. (2010, American Journal of Orthodontics and Dentofacial Orthopedics) compared craniofacial dimensions in 55 mouth-breathing children versus 47 nasal-breathing controls aged six to twelve. Mouth breathers had significantly narrower maxillary arches (12 per cent narrower intermolar width), longer lower face height, steeper mandibular plane angles and more retrognathic mandibles. Linder-Aronson and Backstrom (1960, Acta Oto-Laryngologica) first documented that adenoidectomy (restoring nasal breathing) allowed partial reversal of these changes if performed before age eight.

This is because nasal breathing generates negative pressure in the nasal cavity that acts as a functional matrix (Moss functional matrix theory) stimulating lateral maxillary growth. The tongue rests against the palate during nasal breathing, applying continuous gentle force that widens the palatal vault. Mouth breathing moves the tongue to the floor of the mouth, removes palatal pressure and allows the buccinator muscles to compress the maxilla inward. The narrowed maxilla reduces nasal cavity volume, further impeding nasal airflow and reinforcing the mouth-breathing pattern.

The prevalence of mouth breathing in children is estimated at ten to twenty-five per cent in developed countries (Abreu et al., 2008, International Journal of Pediatric Otorhinolaryngology). Contributing factors include allergic rhinitis (increasingly prevalent), adenoid hypertrophy, nasal congestion from dairy sensitivity and indoor allergen exposure. Orthodontic treatment for the resulting malocclusion (global market $9.2 billion in 2023, Grand View Research) addresses the consequences of mouth breathing, not the cause.

Mudarri and Fisk (2007), in an analysis commissioned by the Lawrence Berkeley National Laboratory, estimated that 47 per cent of US homes had visible dampness or mould growth. The WHO (2009) European review found similar prevalence across the continent. Fisk et al. (2007) meta-analysed 33 studies and found that building dampness and mould were associated with a 30 to 50 per cent increase in respiratory symptoms and asthma outcomes.

This is because moulds produce mycotoxins: secondary metabolites evolved to suppress competing organisms. Trichothecenes (from Stachybotrys), ochratoxin A (from Aspergillus and Penicillium) and aflatoxins are protein synthesis inhibitors, immunosuppressants and carcinogens respectively. These compounds become airborne on mould spores and fragment particles. When inhaled, they deposit on the pulmonary epithelium and are absorbed into the bloodstream. Ochratoxin A has been demonstrated to cross the blood-brain barrier and accumulate in brain tissue (Sava et al. 2006).

The standard medical response to mould exposure is to treat symptoms: inhalers for wheezing, antihistamines for congestion, antidepressants for the neurological symptoms. The source is rarely investigated. Building inspections do not routinely test for mycotoxins. Insurance policies routinely exclude mould remediation. The result is that millions of people live in buildings actively poisoning them through their respiratory system while treating the downstream symptoms as separate, unrelated medical conditions.

The EPA's Total Exposure Assessment Methodology (TEAM) studies (1987) measured personal exposure to volatile organic compounds in 600 residents across six US cities. Indoor concentrations of VOCs were consistently two to five times higher than outdoor levels, regardless of whether the home was in an industrial or rural area. Subsequent EPA reports (2023) confirmed that indoor levels of many pollutants remain two to five times higher than outdoor levels, with peak exposures during and immediately after activities such as cooking, cleaning and painting reaching one hundred times outdoor concentrations.

This is because modern buildings are designed for energy efficiency, with increasing airtightness (reduced air changes per hour) that traps pollutants emitted from building materials (formaldehyde from MDF, benzene from paints, toluene from adhesives), cleaning products (2-butoxyethanol, limonene), cooking (nitrogen dioxide, particulates, acrolein), personal care products (phthalates, synthetic musks) and furnishings (flame retardants, plasticisers). The average Western adult spends ninety per cent of their time indoors (EPA estimate), meaning their dominant air exposure is to this concentrated indoor mixture.

Chronic obstructive pulmonary disease affects 384 million people globally (GBD 2019). While smoking is the primary cause, the WHO attributes 3.2 million annual deaths to household air pollution (primarily in developing countries using solid fuels). In developed countries, the contribution of indoor pollutant exposure from building materials, cleaning products and cooking emissions to respiratory disease burden is poorly quantified but biologically plausible given the concentration differentials documented by the TEAM studies.

The Bohr effect describes haemoglobin's reduced affinity for oxygen in the presence of carbon dioxide. When CO2 is adequate, haemoglobin releases oxygen readily to working tissues. Chronic hyperventilation, often driven by stress, anxiety, or habitual mouth breathing, depletes arterial CO2 below 35 mmHg. At this level, haemoglobin grips oxygen more tightly and tissue oxygenation paradoxically falls despite adequate blood oxygen saturation.

Lum estimated that hyperventilation syndrome affects 6 to 10% of general practice patients and produces symptoms including light-headedness, brain fog, chest tightness, tingling, fatigue and anxiety, which are frequently misdiagnosed as cardiac or psychiatric conditions. The treatment is not more oxygen. It is restoring CO2 tolerance through slower, nasal, diaphragmatic breathing. The most anxious people breathe the most and oxygenate the least.

Harari et al. (2010) compared craniofacial measurements of 55 habitual mouth breathers (ages 6 to 12) with 47 nasal breathers and found consistent skeletal differences: mouth breathers had significantly longer face height (6 to 8 mm), narrower maxillary arch (3 to 5 mm), more retrognathic mandibles (shorter by approximately 10 mm) and steeper mandibular plane angles. The narrower maxillary arch creates a gothic (high-arched) palate that encroaches on the nasal cavity from below.

This is because the tongue is the natural palatal expander. During nasal breathing, the tongue rests against the palate, exerting continuous lateral force that widens the maxilla and creates space for erupting teeth. Mouth breathing drops the tongue posture, removing this force. The cheek muscles, no longer counterbalanced by the tongue, compress the dental arch inward. The mandible adapts to the altered tongue posture by growing downward rather than forward. This pattern is established in childhood and becomes permanent once the sutures fuse.

The consequences cascade beyond appearance. A narrow maxilla means a narrow nasal floor, restricting nasal airflow and reinforcing the mouth-breathing habit. A retrognathic mandible positions the tongue base closer to the posterior pharyngeal wall, reducing airway diameter and predisposing to obstructive sleep apnoea. The modern epidemic of sleep-disordered breathing, orthodontic crowding and "weak chins" is not genetic variation. It is an adaptive response to chronic oral breathing during the critical craniofacial growth window.

The US Environmental Protection Agency (EPA, 2022) states that indoor air pollutant concentrations are typically 2-5 times higher than outdoor levels, and occasionally exceed 100 times outdoor concentrations. Indoor pollutant sources include volatile organic compounds (VOCs) from furniture, carpets, cleaning products and building materials; particulate matter from cooking (especially gas stoves); formaldehyde off-gassing from pressed wood products; and flame retardant chemicals applied to upholstery and textiles.

This is because modern buildings are designed for energy efficiency, with sealed envelopes that trap pollutants inside. Air exchange rates in modern homes are typically 0.3-0.5 air changes per hour, compared to 2-3 in pre-1970s housing. Simultaneously, the chemical burden of indoor environments has increased: a typical modern home contains 300-500 synthetic chemicals that were absent from human living spaces before the 1950s. The EPA itself ranks indoor air quality among the top five environmental health risks.

The average person in the UK and US now spends 90% of their time indoors. This means the primary air supply for the human respiratory system is air that passes through synthetic carpets, adhesives, flame retardants and cleaning chemical residues before reaching the lungs. Traditional dwellings (stone, timber, earth, thatch) contained none of these synthetic pollutant sources and maintained high air exchange rates through natural ventilation. The move indoors has not simply reduced outdoor air exposure; it has replaced it with a more concentrated chemical atmosphere.

The US EPA Total Exposure Assessment Methodology (TEAM) study found that indoor concentrations of common organic pollutants were consistently 2 to 5 times higher than outdoor levels, regardless of whether homes were located in rural or industrial areas. For some volatile organic compounds (VOCs), indoor levels exceeded outdoor by more than ten times. The WHO estimates that 3.2 million deaths per year are attributable to household air pollution globally.

This is because modern buildings are designed for energy efficiency: sealed windows, mechanical ventilation, insulation and draught-proofing. These features trap pollutants from cleaning products, synthetic furnishings, gas cooking appliances, personal care products and building materials. Off-gassing from particle board, carpet, paint and flame retardants creates a continuous low-level chemical exposure. Formaldehyde alone is emitted by pressed-wood products, permanent-press fabrics and certain insulation foams.

The human respiratory system evolved in open air with continuous ventilation. For the entire history of the species, breathing occurred outdoors or in structures with significant air exchange. The sealed indoor environment is an evolutionary novelty. The lungs are not a passive filter. They are an absorptive membrane with a surface area of approximately 70 square metres, actively transferring every inhaled chemical directly into the bloodstream. Five times the outdoor pollution concentration applied to a 70-square-metre absorption surface for 21 of 24 hours per day is the respiratory reality of modern indoor living.