Environmental factors such as indoor air quality, noise, radiation and chemical exposures affect the body continuously and often without notice. Many of these factors are modifiable, but their significance is systematically underestimated compared to lifestyle factors. Here we review the key environmental health factors and what is known about them based on research evidence.

Indoor Carbon Dioxide

Indoor carbon dioxide (CO₂) levels rise quickly when people occupy a space with inadequate ventilation. Outdoor CO₂ concentration is approximately 420 ppm. In typical office and residential spaces, levels range between 600 and 1500 ppm, but poorly ventilated bedrooms and meeting rooms can exceed 2000 ppm.

Controlled experimental studies have shown that cognitive performance, particularly decision-making and strategic thinking, begins to decline already at 1000 ppm. Studies have found that office workers' cognitive performance was significantly better in well-ventilated spaces compared to standard conditions. The effect follows a dose-response pattern: the higher the concentration, the more pronounced the impairment.

In practice, this means it is worth keeping enhanced bedroom ventilation running overnight. Workspaces should be actively ventilated. A CO₂ monitor is a good tool for assessing your own exposure.

Indoor Humidity

The optimal range for indoor relative humidity is between 40% and 60%. Health consequences arise at both extremes, and the dominant problem varies by climate and season.

Low Humidity. In cold climates, indoor air during winter months is typically very dry, often falling below 20% relative humidity. Dry air impairs mucosal defence mechanisms. The ciliary function of the nasal and airway mucosa slows down, reducing pathogen clearance, and dry indoor air has been associated with increased incidence of respiratory infections during winter months. Dry air also worsens atopic dermatitis symptoms, increases ocular dryness and can impair vocal cord function.

A humidifier in the bedroom or home office is a simple solution, though the device requires regular cleaning to prevent microbial growth. Moisture-absorbing materials such as wood and textiles naturally buffer indoor humidity fluctuations.

High Humidity. In warm or tropical climates, and in poorly ventilated spaces anywhere, indoor humidity frequently exceeds 60%. Persistently high humidity creates conditions for several health-relevant problems. Dust mite populations thrive above 50% relative humidity, and dust mites are the single most important source of indoor allergens globally, driving allergic rhinitis and asthma. Mould growth accelerates on surfaces and within building materials even without overt water damage. Elevated humidity also increases VOC off-gassing from furnishings and building materials, compounding chemical exposure. Bacterial proliferation in damp environments can further degrade indoor air quality.

Beyond respiratory and allergic effects, high humidity impairs the body's ability to dissipate heat through evaporation, increasing the risk of heat-related illness. This is particularly relevant for vulnerable populations such as the elderly, young children and individuals with cardiovascular disease.

Practical measures include using a dehumidifier or air conditioning to keep relative humidity below 60%, ensuring adequate ventilation (especially in bathrooms, kitchens and basements), promptly addressing any moisture intrusion and avoiding drying laundry indoors without ventilation. A hygrometer is a cheap and useful tool for monitoring indoor humidity levels.

Volatile Organic Compounds and Indoor Chemicals

Indoor air contains hundreds of volatile organic compounds, commonly known as VOCs. These typically originate from sources such as building materials, paints, adhesives, and new furniture.

While these compounds are most significant in professional settings where exposure levels are high, they are less of a concern in standard environments. For those without specific occupational exposure, the most effective ways to improve air quality are to quit smoking and ensure proper ventilation for one to three months following a renovation or the purchase of new furniture.

Burning candles indoors does release particulate matter and polycyclic aromatic hydrocarbons (PAHs), which can irritate the respiratory system and lungs. However, in most cases, these concentrations remain low and relatively insignificant for overall health.

Outdoor Fine Particulate Matter

Fine particulate matter (PM2.5, particles less than 2.5 micrometres in diameter) is one of the most significant environmental health risks globally. According to the WHO, air pollution causes an estimated 4.2 million premature deaths annually. PM2.5 particles penetrate the alveoli and enter the bloodstream, where they cause systemic inflammation, endothelial dysfunction and oxidative stress.

The association between fine particulate exposure and cardiovascular mortality, lung cancer, asthma exacerbations and neurodegenerative diseases has been convincingly demonstrated in large cohort studies. The dose-response relationship is linear with no clear threshold, meaning that even in relatively clean air, lower exposure is better than higher.

Local sources of fine particulates include residential wood burning, vehicle exhaust and street dust. Living near heavily trafficked roads has been consistently associated with increased cardiovascular disease risk. An air purifier with a HEPA filter effectively reduces indoor particle concentrations. Exercising in the immediate vicinity of busy roads increases exposure.

Microbial Diversity and Environmental Microbial Exposure

The human immune system has evolved in constant interaction with environmental microbes. According to the biodiversity hypothesis, reduced exposure to diverse environmental microbiota is one of the key factors behind the rising prevalence of allergic and autoimmune diseases. A Finnish Karelia study demonstrated that children living in rural areas with abundant environmental microbial exposure had a fraction of the allergy risk compared to children of the same age on the other side of the border, living in a more urbanised and hygienic environment.

In practical terms, spending time in nature, encountering soil microbes (gardening, walking in forests) and having pets are likely beneficial for immune system development and maintenance. Studies have shown that the microbial diversity in homes with dogs is greater than in homes without them.

Excessive hygiene and routine use of antimicrobial cleaning agents in the home is not justified for healthy individuals. Regular soap and water are sufficient for hand hygiene. Widespread use of antibacterial products can disrupt the skin's normal microbiota and promote resistance development.

Indoor microbial damage is a different matter. Mould growth in building materials caused by moisture damage exposes occupants to airway irritation symptoms and exacerbates asthma. The evidence for a causal relationship with new-onset asthma is moderate.

Noise

According to WHO environmental noise guidelines, traffic noise should be below 53 dB outdoors and below 45 dB at night. Noise affects health not only by disrupting sleep but also by activating the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis.

In large European cohort studies, traffic noise has been associated with increased risk of myocardial infarction, stroke and hypertension in a dose-response manner. The effect of noise is partly mediated through sleep disturbance, but daytime noise exposure also raises cortisol levels and blood pressure. Childhood noise exposure has been linked to impaired learning capacity and reading ability.

Practical measures: the bedroom should be as quiet as possible. Window insulation, thick curtains and positioning the bedroom on the quieter side of the dwelling are potential solutions. Earplugs are also an effective option. Reducing long-term noise exposure is more important than avoiding occasional noise. Hearing protection should also be considered at concerts and when using motorised vehicles.

Radiation

Radon. Radon is an odourless and invisible radioactive gas and is the most important risk factor for lung cancer after smoking. It originates naturally from uranium in the soil and enters indoor air through building foundations. Radon levels tend to be higher in areas with glacial ridges and granite bedrock.

In newer construction, radon mitigation (radon piping or extraction systems in the subfloor) is typically required by building codes in many countries. Reference values for indoor radon vary, but 200 to 300 Bq/m³ is a common range. In well-executed new builds, radon is generally not a problem.

Radon is particularly relevant in certain situations. Older homes built before modern radon codes, where no mitigation was installed during construction, represent the highest risk group. Basement spaces and ground-contact rooms with regular occupancy can expose residents to elevated levels. Areas with glacial ridge geology or gravel-based soil are geologically high-risk zones. Changes in ventilation, such as tightening the building envelope during energy retrofits without compensatory fresh air intake, can significantly raise levels in a previously safe building. Ground-floor apartments may also be affected, though levels in multi-storey buildings are generally lower.

The only way to determine your home's radon level is to measure it, typically with a radon test kit placed for a minimum of two months during the heating season. National radiation safety authorities in most countries provide measurement services and regional radon maps. If levels exceed the reference value, remediation options include radon extraction systems, subfloor sealing and improved ventilation.

UV Radiation. Solar ultraviolet radiation is the most important risk factor for skin cancer. The association is well established for both melanoma and non-melanoma skin cancers. Cumulative exposure and childhood sunburns in particular increase risk. UV protection is warranted through the year anytime sun exposure is relevant. UV index guides the level of protection. Reapplying sunscreen and choosing high SPFs are essential for staying protected.

Microplastics

Microplastics (less than 5 mm in diameter) and nanoplastics (less than 1 µm) are ubiquitous in the environment: in water, food, air and human tissues. They have been detected in the placenta, blood, lung tissue and atherosclerotic plaques. A 2024 study published in the New England Journal of Medicine drew particular attention: microplastics found in carotid endarterectomy plaques were associated with a higher risk of cardiovascular events during follow-up.

The evidence on microplastics is evolving. Concerns include chemicals adsorbed to the particle surface (phthalates, bisphenol A, flame retardants), inflammatory response activation and potential effects on intestinal mucosal permeability. Animal studies have demonstrated changes in the gut microbiota and elevated inflammatory markers.

Reducing exposure is reasonable on the basis of the precautionary principle, even though the precise health effects in humans are not yet comprehensively understood. Practical measures include filtering drinking water (activated carbon filter or reverse osmosis), avoiding plastic food containers and especially heating food in plastic, favouring natural fibre textiles (washing synthetic textiles releases microplastics), and drinking from glass or steel vessels instead of plastic bottles.

In saunas and hot environments, synthetic materials (polyester towels, synthetic seat covers) should be avoided, as high temperatures accelerate microplastic release. A cotton towel or linen sauna cloth is a more sensible choice.

Water Filtration

In most developed countries, tap water is safe to drink, but a water filter is a sensible and affordable way to reduce exposure to chlorine compounds and other residues.

An activated carbon filter is the best-studied and most practical option for home use. Activated carbon effectively removes chlorine and its by-products (THM compounds), volatile organic compounds (VOCs), pesticide residues and long-chain PFAS compounds (such as PFOA and PFOS). Fine-pore activated carbon block filters also remove microplastic particles. A limitation of activated carbon is that removal efficiency for short-chain PFAS compounds is variable and often insufficient. Additionally, filter efficiency decreases over time, so regular replacement according to the manufacturer's instructions is essential: an overloaded filter can release accumulated contaminants back into the water.

Reverse osmosis (RO) is the most effective filtration method: it removes virtually all contaminants, including short-chain PFAS compounds, with over 94% efficiency. However, RO filtration also removes beneficial minerals (calcium, magnesium). It is justified primarily in situations where the water is known to contain elevated PFAS concentrations or other specific contaminants.

In practice, a tap-mounted activated carbon filter is sufficient for most households. It improves the taste of water, removes chlorine and its by-products, and reduces exposure to most organic contaminants. When choosing a filter, it is worth paying attention to certifications (such as NSF standards) and replacement intervals.

Light and Circadian Rhythm

The timing of light exposure is one of the most powerful regulators of the circadian rhythm. Bright light containing blue wavelengths in the morning promotes alertness and synchronises the internal clock. Conversely, blue light exposure in the evening delays melatonin secretion and impairs sleep quality. Filtering blue light in the evening (device night mode) is physiologically justified, although the effect on sleep is individual and depends on total light exposure. A bright light therapy lamp (10,000 lux, 20 minutes in the morning) is an evidence-based method for preventing seasonal affective disorder and supporting circadian rhythm regulation.

Ambient Heat

Extreme heat is one of the deadliest environmental hazards globally, and its relevance is increasing with climate change. The 2003 European heat wave caused an estimated 70,000 excess deaths.

The pathophysiology is well characterised. In sustained high temperatures, the cardiovascular system faces increased workload as cardiac output rises to support peripheral vasodilation and heat dissipation. Sweating leads to volume depletion and haemoconcentration, increasing blood viscosity and thrombotic tendency. These mechanisms explain why the majority of excess deaths during heat waves are cardiovascular in nature. Risk is highest among the elderly, individuals with pre-existing cardiovascular disease, and those using medications that impair thermoregulation, including diuretics, beta-blockers, anticholinergics and antipsychotics.

Sleep quality deteriorates significantly when ambient temperature exceeds approximately 24°C. Because sleep is a critical recovery period for the cardiovascular and immune systems, the combination of heat exposure and impaired sleep creates a compounding risk, and this partly explains why heat wave mortality increases with each consecutive hot night. Studies on university students in non-air-conditioned dormitories during heat waves have demonstrated measurable declines in cognitive performance, including reaction time and working memory.

Practical measures include maintaining access to a cool indoor environment during heat waves (even a few hours of cooling per day is protective), staying well hydrated, avoiding exertion during peak heat hours, and using external shading such as curtains or shutters to reduce solar gain. For those in chronically hot climates, air conditioning is the single most effective intervention for reducing heat-related mortality, though fans can provide meaningful benefit at moderate temperatures when humidity is not too high. Long-term, building design choices such as ventilation, insulation, orientation and shading significantly affect indoor heat exposure.

Voluntary Thermal Exposures

Beyond ambient temperature as a risk factor, deliberate thermal exposures have distinct health implications.

Cold exposure activates brown adipose tissue and increases energy expenditure. The acute effects of cold water immersion, such as noradrenaline release and changes in inflammatory markers, have been documented. However, evidence for longer-term health effects is limited and based primarily on observational studies.

Heat exposure through sauna bathing is better studied. The Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD) has demonstrated a dose-response association between sauna bathing frequency and cardiovascular mortality, sudden cardiac death, dementia risk and all-cause mortality. These findings are from observational studies, so a direct causal relationship cannot be concluded, but biological mechanisms (heat shock protein induction, improved vascular endothelial function, blood pressure reduction) support a causal interpretation.

Summary

Environmental health is a domain that is often overlooked despite its significance. The most easily implementable measures are often simple, and managing environmental risks does not typically require major investments. It does, however, require awareness of what you are exposed to.