Nitrogen dioxide pollution exposure is associated with olfactory dysfunction in older U.S. adults

Emissions from modern engines induce distinct effects in human olfactory mucosa cells, depending on fuel and aftertreatment

Ultrafine particles (UFP) with a diameter of ≤0.1 μm, are contributors to ambient air pollution and derived mainly from traffic emissions, yet their health effects remain poorly characterized. The olfactory mucosa (OM) is located at the rooftop of the nasal cavity and directly exposed to both the environment and the brain. Mounting evidence suggests that pollutant particles affect the brain through the olfactory tract, however, the exact cellular mechanisms of how the OM responds to air pollutants remain poorly known. Here we show that the responses of primary human OM cells are altered upon exposure to UFPs and that different fuels and engines elicit different adverse effects. We used UFPs collected from exhausts of a heavy-duty-engine run with renewable diesel (A0) and fossil diesel (A20), and from a modern diesel vehicle run with renewable diesel (Euro6) and compared their health effects on the OM cells by assessing cellular processes on the functional and transcriptomic levels. Quantification revealed all samples as UFPs with the majority of particles being ≤0.1 μm by an aerodynamic diameter. Exposure to A0 and A20 induced substantial alterations in processes associated with inflammatory response, xenobiotic metabolism, olfactory signaling, and epithelial integrity. Euro6 caused only negligible changes, demonstrating the efficacy of aftertreatment devices. Furthermore, when compared to A20, A0 elicited less pronounced effects on OM cells, suggesting renewable diesel induces less adverse effects in OM cells. Prior studies and these results suggest that PAHs may disturb the inflammatory process and xenobiotic metabolism in the OM and that UFPs might mediate harmful effects on the brain through the olfactory route. This study provides important information on the adverse effects of UFPs in a human-based in vitro model, therefore providing new insight to form the basis for mitigation and preventive actions against the possible toxicological impairments caused by UFP exposure.

The Impact of Ambient and Wildfire Air Pollution on Rhinosinusitis and Olfactory Dysfunction

PURPOSE OF REVIEW

With increasing industrialization, exposure to ambient and wildfire air pollution is projected to increase, necessitating further research to elucidate the complex relationship between exposure and sinonasal disease. This review aims to summarize the role of ambient and wildfire air pollution in chronic rhinosinusitis (CRS) and olfactory dysfunction and provide a perspective on gaps in the literature.

RECENT FINDINGS

Based on an emerging body of evidence, exposure to ambient air pollutants is correlated with the development of chronic rhinosinusitis in healthy individuals and increased symptom severity in CRS patients. Studies have also found a robust relationship between long-term exposure to ambient air pollutants and olfactory dysfunction. Ambient air pollution exposure is increasingly recognized to impact the development and sequelae of sinonasal pathophysiology. Given the rising number of wildfire events and worsening impacts of climate change, further study of the impact of wildfire-related air pollution is a crucial emerging field.

The impact of race on olfaction: A systematic review and meta-analysis

BACKGROUND

Many studies have identified a higher degree of Olfactory Dysfunction (OD) in Black patients compared to White patients. This study aims to analyze olfactory outcomes in different races.

METHODS

The PubMed, Scopus, and CINAHL databases were searched from inception to September 5, 2022, for English-language articles documenting self-reported and psychophysical OD stratified by race. A meta-analysis of proportions, comparison of weighted proportions, and comparison of means were performed in MedCalc 20.218. In the quantitative analysis, 79,297 patients were included, comprising 79.3% Whites, 16.1% Blacks, and 4.6% Hispanics.

RESULTS

A total of 14 studies were meta-analyzed. The prevalence of self-reported OD in Hispanic, White, and Black patients was 19.5% (95% CI, 16.6% to 22.6%), 17.2% (95% CI, 10.5% to 25.0%), and 13.9% (95% CI, 9.3% to 19.2%), respectively (p < 0.0007). The prevalence of psychophysical OD in Black, White, and Hispanic patients was 30.3% (95% CI, 24.2% to 36.9%), 24.2% (95% CI, 20.1% to 28.5%), and 18.4% (95% CI, 16.3% to 20.7%), respectively (p < 0.0001). Blacks reported a greater extent of unrecognized OD compared to Whites, with a difference of 16.5% (95% CI, 15.0% to 17.9%) versus 5.8% (95% CI, 3.4% to 8.0%), respectively (p < 0.0001). Hispanic rates of self-reported OD and psychophysical OD were not statistically different.

CONCLUSIONS

Our findings suggest that Blacks have the highest rate of psychophysical OD and are more likely to underreport their awareness compared to Whites.

Ambient Air Pollutants and Olfaction among Women 50-79 Years of Age from the Sister Study

BACKGROUND

Poor olfaction is common in older adults and may have profound adverse implications on their health. However, little is known about the potential environmental contributors to poor olfaction.

OBJECTIVE

We investigated ambient fine particulate matter [PM ≤ 2.5 μ m in aerodynamic diameter (PM 2.5 )] and nitrogen dioxide (NO 2 ) in relation to poor olfaction in middle-aged to older women.

METHODS

The Sister Study is a nationwide cohort of 50,884 women in the United States with annual average air pollutant exposures estimated based on participants' residences from enrollment (2003-2009) through 2017. This analysis was limited to 3,345 women, 50-79 years of age as of January 2018, who completed the Brief Smell Identification Test (B-SIT) in 2018-2019. Poor olfaction was defined as a B-SIT score of ≤ 9 in the primary analysis. We conducted multivariable logistic regressions, accounting for covariates and study sampling design.

RESULTS

Overall, we found little evidence for associations of air pollutants with poor olfaction. The odds ratio (OR) and 95% confidence interval (CI) of poor olfaction for each interquartile range (IQR) increment of air pollutants in 2006 were 1.03 (95% CI: 0.91, 1.17) for PM 2.5 (per 3.3   μ g / m 3 ) and 1.08 (95% CI: 0.96, 1.22) for NO 2 (per 5.7  ppb ). Results were similar in the analyses using the most recent (2017) or the cumulative average (2006-2017) air pollutant exposure data. Secondary analyses suggested potential association in certain subgroups. The OR per IQR was 1.35 (95% CI: 1.11, 1.65) for PM 2.5 among younger participants (< 54.2 years of age) and 1.87 (95% CI: 1.29, 2.71) for NO 2 among current smokers.

DISCUSSION

This study did not find convincing evidence that air pollutants have lasting detrimental effects on the sense of smell of women 50-79 years of age. The subgroup analyses are exploratory, and the findings need independent confirmation. https://doi.org/10.1289/EHP12066.

Acute effects of ambient air pollution on daily neurology clinic visits for vertigo: a time-series study in Wuhan, China

This study aimed to disclose the relationship between ambient air pollution and neurology clinic visits (NCVs) for vertigo. A time-series study was conducted to examine relationships between six different criteria air pollutants (SO₂, NO₂, PM_2.5, PM₁₀, CO, and O₃) and daily NCVs for vertigo in Wuhan, China, from January 1st, 2017 to November 30th, 2019. Stratified analyses were computed according to gender, age, and season. A total of 14,749 records of NCVs for vertigo were enrolled in this study. Data showed that the increase in daily NCVs for vertigo corresponding to 10 μg/m³ increase of respective pollutants are: SO₂ (- 7.60%; 95% CI: - 14.25 to - 0.44%), NO₂ (3.14%; 95% CI: 0.23 to 6.13%), PM_2.5 (0.53%; 95% CI: - 0.66 to 1.74%), PM₁₀ (1.32%; 95% CI: - 0.36 to 3.06%), CO (0.00%; 95% CI: - 0.12 to 0.13%), and O₃ (0.90%; 95% CI: - 0.01% to 1.83%). Males were more susceptible to acute exposure to SO₂ and NO₂, compared to females (SO₂: - 11.91% vs. - 4.16%; NO₂: 3.95% vs. 2.92%), whereas the acute effect of O₃ exposure was more significantly obvious in females than males (0.94% vs. 0.87%). Moreover, correlations between daily NCVs for vertigo and acute exposure to SO₂, NO₂, and O₃ were all stronger in individuals under 50 years old (SO₂: - 12.75% vs. - 4.41%; NO₂: 4.55% vs. 2.75%; O₃: 1.27% vs. 0.70%). Short-term exposure to PM_2.5 was more significantly associated with daily NCVs for vertigo in cool seasons (1.62% vs. - 0.68%), while the correlation between CO exposure and daily NCVs for vertigo was stronger in warm seasons (0.21% vs. - 0.03%). Our study demonstrated acute exposure to ambient NO₂ and O₃ positively associated with daily NCVs for vertigo. Acute effects of air pollution on daily NCVs for vertigo varied according to gender, age, and season.

Association of air pollution with olfactory identification performance of São Paulo residents: a cross-sectional study

OBJECTIVE

Exposure to particulate matter of 10 μm or less in diameter (PM₁₀) has been implicated in pulmonary and cardiovascular diseases. However, the effect of PM₁₀ on olfaction has not been well established. We estimated individual acute and chronic PM₁₀ exposure levels in a large Brazilian cohort and related them to the ability to identify odors.

METHODS

Adults from São Paulo (n = 1358) were recruited from areas with different levels of air pollution. To verify individual exposure to air pollution, the averages of 30, 60, 90, 180 and 364 days of PM₁₀ were interpolated to subjects' zip codes using the kriging method. Olfactory identification performance was tested using the University of Pennsylvania Smell Identification Test (UPSIT®). Multiple linear regressions were used to calculate the effect of air pollution on olfactory identification performance, controlling for demographic and other variables that affect the sense of smell.

RESULTS

Acute exposures to PM₁₀ were related to worse UPSIT® scores, including 30- (β =  - 0.94, 95% Confidence Interval [CI]  - 0.98,  - 0.89), 60- (β =  - 1.09, 95% CI = - 1.13,  - 1.04) and 90-day intervals (β =  - 1.06, 95% CI  - 1.10,  - 1.02) (reference for β: 1 µm/m³ increase in PM10 exposure per point decrease in UPSIT® score). Chronic exposures were also associated with worse olfaction for both 180- (β =  - 1.06, 95% CI  - 1.10,  - 1.03) and 364-day (β =  - 0.87, 95% CI  - 0.90,  - 0.84) intervals. As in prior work, men, older, low-income, and low-schooling people demonstrated worse olfactory performance.

CONCLUSION

Acute and chronic exposure to PM₁₀ is strongly associated with olfactory identification performance in Brazilian adults. Understanding the mechanisms which underlie these relationships could help to improve chemosensory function with a large public health impact.

International consensus statement on allergy and rhinology: Olfaction

BACKGROUND

The literature regarding clinical olfaction, olfactory loss, and olfactory dysfunction has expanded rapidly over the past two decades, with an exponential rise in the past year. There is substantial variability in the quality of this literature and a need to consolidate and critically review the evidence. It is with that aim that we have gathered experts from around the world to produce this International Consensus on Allergy and Rhinology: Olfaction (ICAR:O).

METHODS

Using previously described methodology, specific topics were developed relating to olfaction. Each topic was assigned a literature review, evidence-based review, or evidence-based review with recommendations format as dictated by available evidence and scope within the ICAR:O document. Following iterative reviews of each topic, the ICAR:O document was integrated and reviewed by all authors for final consensus.

RESULTS

The ICAR:O document reviews nearly 100 separate topics within the realm of olfaction, including diagnosis, epidemiology, disease burden, diagnosis, testing, etiology, treatment, and associated pathologies.

CONCLUSION

This critical review of the existing clinical olfaction literature provides much needed insight and clarity into the evaluation, diagnosis, and treatment of patients with olfactory dysfunction, while also clearly delineating gaps in our knowledge and evidence base that we should investigate further.

Environmental Air Pollution and Olfactory Decline in Aging

BACKGROUND

Olfactory impairment is increasingly common with older age, which may be in part explained by cumulative effects of exposure to inhaled toxins. However, population-based studies investigating the relationship between air pollution and olfactory ability are scarce.

OBJECTIVES

We aimed to investigate associations between exposure to common air pollutants and longitudinal change in odor identification.

METHODS

Our study of 2,468 participants (mean age=72.3y; 61.1% female), of which 1,774 participants (mean age=70.5y; 61.9% female) had at least two olfactory assessments over 12 y of follow-up from the Swedish National Study on Aging and Care in Kungsholmen (SNAC-K), Stockholm, Sweden. Participants were free from cognitive impairment and neurodegenerative disease at baseline. Odor identification ability was assessed with Sniffin' Sticks. Change in olfactory performance was estimated with linear mixed models. Exposure to two major airborne pollutants [particulate matter with aerodynamic diameter ≤2.5μm (PM2.5) and nitrogen oxides (NOx)] for the 5 y preceding baseline was assessed using spatiotemporal dispersion models for outdoor levels at residential addresses.

RESULTS

Participants showed significant decline in odor identification ability for each year in the study {β=-0.20 [95% confidence interval (CI): -0.22, 0.18; p<0.001]}. After adjustment for all covariates, residents of third [β=-0.09 (95% CI: -0.14, -0.04; p<0.001)] and fourth [β=-0.07 (95% CI: -0.12, -0.02; p=0.005)] exposure quartiles of PM2.5 had faster rates of olfactory decline than residents from the first quartile. Similar results were observed for the third [β=-0.05 (95% CI: -0.10, -0.01; p=0.029)] and fourth [β=-0.07 (95% CI: -0.11, -0.02; p=0.006) quartiles of NOx].

DISCUSSION

Our results suggest an association between air pollution exposure and subsequent olfactory decline. We speculate that cumulative effects of airborne pollutants on the olfactory system may be one underlying cause of olfactory impairment in aging. https://doi.org/10.1289/EHP9563.

Environmental triggers of Parkinson's disease - Implications of the Braak and dual-hit hypotheses

Idiopathic Parkinson's disease (PD) may take decades to develop, during which many risk or protective factors may come into play to initiate the pathogenesis or modify its progression to clinical PD. The lack of understanding of this prodromal phase of PD and the factors involved has been a major hurdle in the study of PD etiology and preventive strategies. Although still controversial, the Braak and dual-hit hypotheses that PD may start peripherally in the olfactory structures and/or the gut provides a theoretical platform to identify the triggers and modifiers of PD prodromal development and progression. This is particularly true for the search of environmental causes of PD as the olfactory structures and gut are the major human mucosal interfaces with the environment. In this review, we lay out our personal views about how the Braak and dual-hit hypotheses may help us search for the environmental triggers and modifiers for PD, summarize available experimental and epidemiological evidence, and discuss research gaps and strategies.

Relationship between socioeconomic status, exposure to airborne pollutants, and chronic rhinosinusitis disease severity

BACKGROUND

Air pollution directly interacts with airway mucosa, yet little is known about how pollutants affect upper airway inflammation. Studies have shown increased incidence of chronic rhinosinusitis (CRS), rhinitis, and asthma in areas with higher traffic pollution, and these neighborhoods are often associated with lower socioeconomic status (SES). The Area Deprivation Index (ADI) assesses neighborhood-level SES by zip code. The purpose of this study was to assess the relationship between SES and exposure to inhaled pollutants and CRS disease severity.

METHODS

CRS patients with and without nasal polyps (CRSwNP and CRSsNP, respectively) were identified (total patients = 234; CRSwNP patients = 138; CRSsNP patients = 96). Pollutant concentrations, including particulate matter 2.5 (PM_2.5 ), black carbon (BC), and nitrogen dioxide (NO₂ ), were measured at 70 sites within the defined countywide sites and used to estimate patient exposures. SES was measured by ADI state deciles. Disease severity metrics included the modified Lund-Mackay score (LMS), the need for systemic steroids, and functional endoscopic sinus surgery (FESS). Associations were analyzed and identified using linear, logistic, and Poisson multivariable regression.

RESULTS

The distribution of CRSsNP and CRSwNP patients across ADI state deciles was similar. ADI, however, was a predictor of exposure to airborne pollutants (PM_2.5 , BC, and NO₂ ) with a 1.39%, 2.39%, and 2.49% increase in PM_2.5 , BC, and NO₂ per increasing decile increment (p < 0.0001), respectively, which demonstrated a direct correlation between deprived neighborhoods and higher levels of exposure to PM_2.5 , BC, and NO₂ with an increase in pollutant levels per increase in ADI decile. Furthermore, ADI was a predictor for increased steroid treatment.

CONCLUSION

Lower SES predicted higher exposure to air pollution and increased disease severity in patients with CRS as demonstrated by the increased need for steroid treatment.

Air Pollution: A Neglected Risk Factor for Dementia in Latin America and the Caribbean

The risk of dementia and Alzheimer's disease in Latin America and the Caribbean (LAC) rises with increasing age and polluted air. Currently, at least 172 million people breathe unhealthy levels of air pollution in LAC countries. Several cohort studies have indicated that air pollution increases the risk of developing dementia and neurodegenerative diseases, but the mechanisms underlying the association are still not clear. Air pollution causes and aggravates five established risk factors for dementia (obesity, hypertension, stroke, diabetes mellitus, and heart diseases) and is linked to three other risk factors (physical inactivity, cognitive inactivity, and depression). Some of these risk factors could be mediating the association between air pollution and dementia. Reducing the risks for dementia is crucial and urgently needed in LAC countries. There is room for improving air quality in many urban areas in the LAC region and other low- and middle-income countries (LMICs), a routealready explored by many urban areas in developing regions. Moreover, reducing air pollution has proved to improve health outcomes before. In this article, we propose that despite the ongoing and valid scientific discussion, if air pollution can or cannot directly affect the brain and cause or aggravate dementia, we are ready to consider air pollution as a potentially modifiable risk factor for dementia in LAC and possibly in other LMICs. We suggest that controlling and reducing current air pollution levels in LAC and other LMIC regions now could strongly contribute.

Association between social determinants of health and olfactory function: a scoping review

BACKGROUND

Social determinants of health (SDoH) include the socioeconomic, demographic, and social conditions that influence differences in health status among individuals and groups. The impact of these conditions on olfactory function remains poorly understood. In this scoping review, we systematically review the available literature to synthesize the association between SDoH and olfactory function.

METHODS

In accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Review (PRISMA-ScR) guidelines, we performed systematic search queries in PubMed, Embase, and Ovid databases and categorized articles according to themes that emerged regarding SDoH. The primary outcomes included self-reported and objective measurements of smell.

RESULTS

We identified 722 unique references that underwent title and abstract review by two independent reviewers, with 70 articles undergoing full-text review and 57 relevant for data extraction. Six themes emerged in our review, under which we categorized the studies and synthesized respective associations with olfactory function. These include studies exploring socioeconomic status (n = 19, 33%), education status (n = 27, 47%), occupational exposures (n = 26, 46%), racial/ethnic disparities (n = 12, 21%), and lifestyle/behavioral factors (n = 33, 58%).

CONCLUSIONS

Within the context of this scoping review, olfactory dysfunction is significantly more prevalent in patients with lower socioeconomic status, exposure to environmental and occupational toxins, and of minority race/ethnicity, whereas the associations between olfactory dysfunction and education level and lifestyle factors such as smoking and drinking seem to be much more elusive. This review highlights the importance of accounting for SDoH in observational studies examining olfactory outcomes. Given the increased awareness of olfactory loss, special consideration should be given to understanding olfactory dysfunction in the context of these factors.

Olfactory loss and aging: connections with health and well-being

Age-related olfactory dysfunction, or presbyosmia, is a common sensory impairment in aging adults. People in this demographic group with comorbid conditions or exposure to viral, traumatic, or environmental insults remain at the greatest risk for impairment. Several methods for assessing olfaction exist, but they are only available in special settings and require consideration of age, sex, ancestry, and cognition. Perhaps most importantly, olfactory dysfunction has been suggested as an early sign of Alzheimer's and Parkinson's disease and therefore may serve as a tool in the diagnosis and prognosis of these neurodegenerative conditions. Outside of this context, olfactory loss also impacts nutrition, safety, and social relationships, and even predicts mortality itself. This review covers the detection and manifestations of olfactory decline in aging individuals and the myriad ways in which olfactory impairment is connected to their health and well-being.

Neurotoxicity and Chemoreception: A Systematic Review of Neurotoxicity Effects on Smell and Taste

Several different types of exposure have the potential to produce olfactory and gustatory deficits related to neurotoxicity. Although the literature contains relatively few studies of such chemoreceptive dysfunction in the context of toxic exposure, this review explores the strength of such published associations. Several studies collectively demonstrated moderately strong evidence for an association between manganese dust exposure and olfactory deficits. Evidence of associations between individual chemicals, therapeutics, and composites, such as World Trade Center debris, and olfactory and gustatory deficits remains limited or mixed. Further need for controlled studies for clinical management, exposure limits, and policy development is identified.

Severe air pollution links to higher mortality in COVID-19 patients: The "double-hit" hypothesis

OBJECTIVES

In areas of SARS-CoV-2 outbreak worldwide mean air pollutants concentrations vastly exceed the maximum limits. Chronic exposure to air pollutants have been associated with lung ACE-2 over-expression which is known to be the main receptor for SARS-CoV-2. The aim of this study was to analyse the relationship between air pollutants concentration (PM 2.5 and NO2) and COVID-19 outbreak, in terms of transmission, number of patients, severity of presentation and number of deaths.

METHODS

COVID-19 cases, ICU admissions and mortality rate were correlated with severity of air pollution in the Italian regions.

RESULTS

The highest number of COVID-19 cases were recorded in the most polluted regions with patients presenting with more severe forms of the disease requiring ICU admission. In these regions, mortality was two-fold higher than the other regions.

CONCLUSIONS

From the data available we propose a "double-hit hypothesis": chronic exposure to PM 2.5 causes alveolar ACE-2 receptor overexpression. This may increase viral load in patients exposed to pollutants in turn depleting ACE-2 receptors and impairing host defences. High atmospheric NO2 may provide a second hit causing a severe form of SARS-CoV-2 in ACE-2 depleted lungs resulting in a worse outcome.

Associations between Cadmium Exposure and Taste and Smell Dysfunction: Results from the National Health and Nutrition Examination Survey (NHANES), 2011-2014

Background: Cadmium is a ubiquitous environmental pollutant and has been associated with many adverse health outcomes. However, little is known about the effect of cadmium exposure on taste and smell dysfunction. Methods: We used the National Health and Nutrition Examination Survey (NHANES) 2011–2014 to investigate the associations between blood cadmium and taste and smell dysfunction among 5038 adults aged 40–80 years old. Taste and smell dysfunction were defined by questionnaires, examinations, or both criteria. Results: In survey weighted logistic regression models adjusting for age, gender, race/ethnicity, income-to-poverty ratio (IPR), and education, individuals with a blood cadmium level in the highest tertiles had significantly higher odds of having perceived smell dysfunction (odds ratio (OR) = 1.41, 95% confidence interval (CI): 1.08, 1.84), perceived taste dysfunction (OR = 1.48, 95% CI: 1.16, 1.89), and taste dysfunction defined by both self-reported and objectively measured data (OR = 1.46, 95% CI: 1.03, 2.07). After further adjusting for body mass index (BMI), cigarette smoking, and alcohol drinking, consistent results were observed for perceived taste dysfunction (OR = 1.49, 95% CI: 1.10, 2.00), and no significant associations were found between cadmium exposure and other outcomes. Conclusions: Our findings suggest that cadmium exposure is associated with perceived taste dysfunction.

Acute versus Chronic Exposures to Inhaled Particulate Matter and Neurocognitive Dysfunction: Pathways to Alzheimer's Disease or a Related Dementia

An estimated 92% of the world's population live in regions where people are regularly exposed to high levels of anthropogenic air pollution. Historically, research on the effects of air pollution have focused extensively on cardiovascular and pulmonary health. However, emerging evidence from animal and human studies has suggested that chronic exposures to air pollution detrimentally change the functioning of the central nervous system with the result being proteinopathy, neurocognitive impairment, and neurodegenerative disease. Case analyses of aging World Trade Center responders suggests that a single severe exposure may also induce a neuropathologic response. The goal of this report was to explore the neuroscientific support for the hypothesis that inhaled particulate matter might cause an Alzheimer's-like neurodegenerative disease, in order to consider proposed mechanisms and latency periods linking inhaled particulate matter and neurodegeneration, and to propose new directions in this line of research.

Anatomical basis and physiological role of cerebrospinal fluid transport through the murine cribriform plate

Cerebrospinal fluid (CSF) flows through the brain, transporting chemical signals and removing waste. CSF production in the brain is balanced by a constant outflow of CSF, the anatomical basis of which is poorly understood. Here, we characterized the anatomy and physiological function of the CSF outflow pathway along the olfactory sensory nerves through the cribriform plate, and into the nasal epithelia. Chemical ablation of olfactory sensory nerves greatly reduced outflow of CSF through the cribriform plate. The reduction in CSF outflow did not cause an increase in intracranial pressure (ICP), consistent with an alteration in the pattern of CSF drainage or production. Our results suggest that damage to olfactory sensory neurons (such as from air pollution) could contribute to altered CSF turnover and flow, providing a potential mechanism for neurological diseases.

The Search for Environmental Causes of Parkinson's Disease: Moving Forward

It is widely believed that environmental exposures contribute to the vast majority of late-onset sporadic Parkinson's disease (PD), alone or via interactions with genetic factors. The search for environmental causes of PD has however been hampered by lack of understanding the prodromal phase of PD development and the difficulties in exposure assessment during this prolonged period. On the other hand, the existence of this prodromal period, along with an increasingly better understanding of PD prodromal symptoms, provides an exciting opportunity to identify environmental factors that initiate PD pathogenesis and/or modify its progression. For prevention efforts, this prodromal stage is of a major interest. Targeting factors that enter the body via the nose or gut has become even more important since the discovery of α-synuclein aggregates in the enteric and olfactory nervous systems. In this paper, we speculate about novel research hypotheses and approaches that may help us better define the role of environment in PD etiology, especially during its extended and complex prodromal phase.