Environmental factors associated with autism spectrum disorder in Southern Europe: a systematic review

Abstract

The aetiology of autism spectrum disorder (ASD) is heterogeneous and is attributed to the concurrent interaction of a number of genetic and environmental factors. The steady increase in ASD rates in recent years makes the detection and study of environmental risk factors increasingly important. This systematic review identifies potential environmental factors associated with ASD focusing specifically on recent studies conducted in selected Southern European countries. Nine relevant studies were selected for this systematic review. Results indicate that ASD in Southern Europe is associated with a variety of prenatal, perinatal and postnatal environmental factors, including stressful early life events, maternal infection during pregnancy, delivery mode and perinatal complications or breastfeeding problems in the offspring. Specially designed, large population-based birth cohort studies are needed in Southern Europe, to allow precise assessment of potential environmental confounders and elucidate their association with ASD.

Introduction

Autism Spectrum Disorder (ASD) refers to a neurodevelopment condition characterized by difficulties in social communication and interaction, together with restricted and repetitive patterns in behaviors, interests and activities. The term ‘spectrum’ is used because of the heterogeneity in the expression and the severity of ASD symptoms, as well as, in the skills and level of functioning of ASD individuals (Lord et al. 2020). Up to 15% of ASD cases can be linked to a known genetic cause via monogenic syndromes (such as fragile X syndrome, tuberous sclerosis, and Timothy syndrome) (Devlin and Scherer 2012). Interestingly, ASD prevalence has increased significantly in the last decades. According to the 11-site Autism and Developmental Disabilities Monitoring network in the USA, the most recent prevalence was estimated at 1.85% in 2016, compared to 1.68% in 2014 (Maenner et al. 2021, Shaw et al. 2021). According to the Autism Spectrum Disorders in the European Union project (ASDEU, Delobel-Ayoub et al. 2015), the average ASD prevalence in Europe, across 12 countries (Austria, Bulgaria, Ireland, Italy, Poland, Portugal, Romania, Spain, Denmark, Finland, France and Iceland) was estimated at 1.22%. In Greece, the first large-scale epidemiological study (Thomaidis et al. 2020), using 2180 official cases of diagnosed children born between 2008 and 2009, has shown a prevalence of 1.15% ASD at national level with the age of six years and one month, being the mean age of ASD diagnosis.

It is well known that ASD pathogenesis is complex. The discovery of specific genes associated with the condition has provided a useful insight into its biological mechanisms. Evidence, also, suggests that environmental factors lead to diverse phenotypes, depending on the developmental timing of exposure (Kim and Leventhal 2015). Many studies in twins and other family studies, have pointed out the importance of inherited predisposition to the disorder (Hallmayer et al. 2011, Tchaconas and Adesman 2013) and yielded positive findings in specific chromosomal regions (Li et al. 2012), as well as, increases in the Copy Number Variations (CNVs) in simplex ASD families (Kim and Leventhal 2015). It is important to note that there is evidence that Epigenetic mechanisms are thought to be critical in the normal development of the nervous system (Modabbernia et al. 2017) and that some environmental risk factors of ASD might affect neurodevelopment through them.

Compared to a relatively low number of genetic studies on ASD, several studies, including reviews, have investigated the potential role of prenatal, perinatal and postnatal environmental factors in the aetiology of ASD. None of these studies provide sufficient support for a causal role of any single environmental risk factor, in the onset of ASD (Pugsley et al. 2022). The most widely cited environmental risk factors in the aetiology of ASD include psychiatric and neurological conditions, being overweight, inflammation, autoimmune reactions, pregnancy-specific conditions and perinatal complications (Bolte et al. 2019, Gialloreti et al. 2014, Hallmayer et al. 2011, Hertz-Picciotto et al. 2018, Kolevzon et al. 2007, Ng et al. 2017). More specifically, higher parental age (Atsem et al. 2016, Merikangas et al. 2017, Sandin et al. 2016), increased maternal stress (Hecht et al. 2016, Rai et al. 2012), mid-gestational vitamin D deficiency (Lee et al. 2021, Vinkhuyzen et al. 2017), exposure to infections during pregnancy (Al-Haddad et al. 2019), pregnancy complications such as placental diseases or preeclampsia (Bajalan and Alimoradi 2020), delivery date diverged from 40 wk (Persson et al. 2020), low neonatal weight (Chien et al. 2019, Haglund and Källén 2011, Hultman et al. 2002), caesarean section with general anesthesia (Al-Zalabani et al. 2019, Huberman et al. 2019), in combination with absence or inadequate breastfeeding (Persson et al. 2020), early antibiotic treatment of the new-born (Isaksson et al. 2017), or neonatal intensive care unit (NICU) environment conditions (e.g. long stay and increased noise level exposure in incubators) (Zacarías et al. 2018), have been linked with ASD, in the past. The evidence on the possible association between maternal prenatal smoking and the child’s risk for ASD has been conflicting (Hertz-Picciotto et al. 2022), while the link between vaccines and ASD is not supported (Mohammed et al. 2022).

Interestingly, most of the above studies have been conducted in USA, Northern Europe and Asia, whereas, studies conducted in Southern Europe are fewer, recently published and, to the best of our knowledge, not reviewed. Thus, the main purpose of the present study is to provide a synopsis of studies conducted in Southern Europe in the past 10 years, on potential factors associated with the development of ASD and discuss the findings.

Method

Literature search

In order to identify factors associated with ASD in the offspring, an electronic database search using PubMed and Eric was conducted. The keywords used were autism, autism spectrum disorder and perinatal factors, prenatal factors, postnatal factors, environmental factors, or pregnancy and gestational, maternal depression, air pollution, heavy metals, stressful life events, maternal diabetes mellitus, substance use, vitamin D, preeclampsia, maternal infections, parental age, cesarean section, prematurity, birth order, incubators, breastfeeding and early-life antibiotic use. Only studies published in English language from 2010 up to 2021 were included in this review.

Study selection

The review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement. Each publication was first screened through the title and the abstract and, then, careful consideration was given to the full text on the basis of the following inclusion criteria: the study should include Southern Europe countries, it should focus on ASD and it should consider environmental factors associated with ASD. Literature reviews of not relevant research published in the past six years, conference abstracts, meta-analyses referring to research from other countries and animal studies, were excluded from the current review. Overall nine studies from Southern Europe (Greece, Italy, Turkey, France and Spain) met the criteria and hence they were included in the current review. The selection criteria flowchart is shown in Figure 1.

Figure 1.

Flowchart of the literature search and selection.

Study characteristics

The main characteristics and results of the reviewed studies are presented in Table 1. Five of the studies were case-control studies, 1 was cross-sectional, 2 were population-based cohort studies and 1 retrospective study. The number of the participants to the above studies included in the review is ranged from 137 to 2.644. Participants were mothers of children and adolescents with ASD, ASD children and adolescents, their siblings, typically developing children and children with Attention Deficit/Hyperactivity Disorder (ADHD) (control groups).

Table 1.

Summary of characteristics of included studies.

Study	Country	Sample	Study design	Objective	Measures	Results
Cassimos et al. 2016	Greece	273 children with ASD & 273 healthy children, matched for age	Prospective cohort study	To examine perinatal and parental risk factors associated with ASD	Semi-structured interviews	The risk factors associated positively with ASD include: male gender; gestational age, multiple gestations; maternal age and education
Grossi et al. 2016	Italy	45 mothers of ASD children, 68 mothers of TD children 24 siblings of 19 ASD children	Case-control	Assessment the frequency of 27 potential risk factors related to pregnancy and peri-postnatal period	Structured Interview with the mothers of children with ASD and typically developing respectively	Solvents/paints exposure during pregnancy, life stressful events, pregnancy and perinatal complications, breastfeeding deficiency and early antibiotic therapy were associated with ASD in the offspring
Grossi et al. 2018	Italy	249 children (73 children & adolescents: Group A) (35 children & adolescents with ASD: Group A1) (45 siblings of children with ASD: Group B) (96 TD children matched for gender and age: Group C)	Case-control	Assessment the frequency of 27 potential risk factors related to pregnancy and peri-postnatal period	Structured Interview with the mothers of them	Stressful life events during pregnancy are associated with ASD
Avella-Garcia et al. 2016	Spain	2644 mother-child pairs	Population-based birth cohort study	Assessment between prenatal exposure to acetaminophen and neurodevelopmental outcomes at 1 and 5 years old	Structured interviews, BSID, MCSA, CPSCS, CAST, K-CPT	Prenatal acetaminophen exposure was associated with autism symptoms in males and adverse effects on attention-related outcomes for both genders
Perales-Marín et al. 2018	Spain	251 mother-child pairs (53 cases: 47 boys & 6 girls) (198 controls: 100 boys & 98 girls)	Case-control	Evaluation of the relationship between cesarean sections and ASD	Hospital’s patient record database	Cesarean delivery combined with the gender (male) is associated with ASD.
López-Vicente et al. 2019	Spain	2,107 mother-child pairs (5, 8, 14, 18 years old)	Population-based birth cohort study	Association between prenatal 25(OH)D3 concentration and behavior during childhood	Maternal Blood specimen, CAST, CPSCS, SDQ, CBCL, CPRS-R	Prenatal 25(OH)D3 is related to social skills at 5 years of age.
Say et al. 2016	Turkey	280 children in the age range of 3–18 years old (100 ASD) (100 ADHD) (80 TD)	Case-control	Investigation of the shared and non-shared risk factors for ASD, ADHD and TD children	Structured Interview with mothers	Prematurity and maternal stress/depressive mood in pregnancy were common risk factors by ASD and ADHD. Postpartum maternal depressive mood was more specific to ASD, while shorter duration of breastfeeding was probably related to ADHD
Ugur et al. 2019	Turkey	196 children with ASD and 54 healthy children	Case-control	To evaluate the birth order of children with ASD and ways of delivery at birth, together with the phenomenon of reproductive stoppage and the number of siblings in the case families	Turkish version of Autism Behavior Checklist (ABC), Aberrant Behavior Checklist (AbBC) and Childhood Autism Rating Scale scores (CARS)	A significant difference for being the first-born child in the ASD group was found. The rate of difficulties during labour was determined to be higher in the ASD group than that of the control group
Brayette et al. 2019	France	254 children with ASD: age range of 3–15 years old (19 born MPLT, 60 born ET and 175 FT)	Retrospective study	Comparison of the phenotypic characteristics between children with ASD born MLPT, ET and FT (after 38 WGA)	CARS, BSE-R, RRB, WPPSI-III, WPPSI-IV, WISC-IV, EDEI-R, PEP-3, SCEB	Incomplete gestation, i.e. MLPT or ET, has a negative impact on both verbal and nonverbal abilities

GA; Gestational age, ADOS – Autism Diagnostic Observation Schedule-Second Edition, BSID: Bayley Scales of Infant Development, MCSA: McCarthy Scales of Children’s Abilities, CPSCS: California Preschool Social Competence Scale, CAST: Childhood Autism Spectrum Test, K-CPT: Conner’ s Kiddie Continuous Performance Test, SDQ: Strengths and Difficulties Questionnaire, CBCL: Manual for the ASEBA School-Age Forms and Profiles, CPRS-R: Conner’s’ Rating Scales-Revised technical manual, ADI-R: Autism Diagnostic Interview-revised, CARS: Childhood Autism Rating Scale test, AbBC: Aberrant Behavior Checklist, BSE-R: Revised Behavior Summarized Evaluation Scale, RRB: Repetitive and Restricted Behavior scale, Wechsler intelligence scales: WPPSI-III, WPPSI-IV, WISC-IV, EDEI-R: Differential Scales of Intellectual Efficiency-Revised edition, PEP-3: Psychoeducational Profile-third edition, SCEB: Social Cognitive Evaluation Battery.

MLPT: Moderately and Late Preterm (32–36 WGA), ET: Early Term (37–38 WGA), FT: Full Term (>38 WGA).

Results and discussion

Environmental risk factors before and during pregnancy

Parental age

The association between older parental age at conception and neuropsychiatric disorders including ASD in offspring is well documented (Merikangas et al. 2017). In one of the reviewed studies (Cassimos et al. 2016), parents of ASD children were significantly older than controls cases’ parents, but only maternal age over 35 years emerged as a significant risk factor for ASD. Older maternal age was, also, associated with higher education level, delayed childbearing and fertility treatment in the cohort group. These findings are in line with similar studies (see meta-analysis of 27 studies by Wu et al. 2017) which address important health implications (i.e. better understanding of reproductive mechanisms and ASD in men and women) deriving from the significant increase in average maternal and paternal age in the last decades. For example, de novo mutations – a major cause of severe early-onset genetic disorders including ASD – are predominantly of paternal origin and their number increases with advanced paternal age (Wu et al. 2017). Similarly, advanced maternal age is associated with a much higher rate of perinatal and obstetric complications (Glasson et al. 2004), which increase the risk of ASD and are usually caused by other factors, such as possible nutritional deficiencies, metabolic conditions, or autoimmune disorders (Wu et al. 2015).

Birth order together with the phenomenon of reproductive stoppage

Hoffmann et al. (2014) and Wood et al. (2015) point out that having a child with ASD is associated with reproductive stoppage, whereas Grønborg et al. (2015) report contradictory findings on the same issue. In one of the reviewed studies, Ugur et al. (2019) compared ASD group and controls in terms of birth order and found significantly more first-born children in the ASD group. Mothers in the ASD group reported that having a child with ASD did not affect their decision for the next pregnancy, a result not in line with other findings suggesting that the psychosocial implications of having children with ASD may cause fewer children in families by influencing the decision for the next pregnancy (Hoffmann et al. 2014, Wood et al. 2015). However, this specific study is not community-based and involved only simplex families. Evidence on the link between birth order, reproductive stoppage and risk for ASD could have important implications for recurrence risk estimation and genetic counselling.

Maternal stress or stressful life events

In 3 out of the 9 studies reviewed, maternal stress/depressive mood or stressful life events during pregnancy, such as divorce, separation or conjugal conflict, were significantly related to ASD (Grossi et al. 2016, 2018, Say et al. 2016). The timing of gestational stress appears to be important with regard to the effect on the unborn offspring’s risk of developing emotional problems later on (O'Connor et al. 2002). Most of the associations with emotional problems appear to be with stress during late gestation which is a period when the child’s brain is developing rapidly and important changes are occurring (Grossi et al. 2018). Maternal distress during pregnancy increases plasma levels of cortisol and corticotrophin releasing hormone in both the mother and the foetus. This may result in insulin resistance and behavior disorders in the offspring such as attention and learning deficits, generalized anxiety and depression (Weinstock 2008). In conclusion, adverse experiences during the prenatal period have been showed to induce significant effects on neurology, metabolism and psychology that can persist across the life span. The findings suggest that an intervention aiming at minimizing the number of stressors that a woman experiences during pregnancy, could have an effect in reducing the risk of the offspring developing autistic traits (Grossi et al. 2016).

Vitamin D

Vitamin D modulates the central nervous system function and has neuroprotective properties and antioxidant effects. Its deficiency is a public health problem worldwide and, if it occurs during critical periods of development, increases the risk of different psychiatric and neurological disorders (DeLuca et al. 2013). López-Vicente et al. (2019) conducted a survey in five regions across Spain and measured 25(OH)D₃ maternal concentrations in plasma during pregnancy. The study population included mothers (94% born in Spain, mean age = 30.7 years old, SD = 4.2) and their children (50.8% boys, 56.4% firstborn). Behavioral problems, ADHD or ASD symptoms and social competence in children, were assessed using different tools and at different ages. Low levels of vitamin D in younger women with pre-pregnancy overweight or obesity, primiparous, active smoking or partners smoking at home during pregnancy were found. Furthermore, the study has shown that children who are prenatally exposed to deficient concentrations of 25(OH)D₃ tend to show more ASD symptoms, more behavioral problems and less social competence at the age of 5, compared to their peers exposed to higher concentrations of vitamin D. The group of children with low concentrations of 25(OH)D₃ during gestation appears to have more ADHD symptoms at the age of 8 years and more behavioural problems at the age of 8 and 14 years. However, this pattern was not detected at the age of 18 years. It is worth pointing out that a number of factors including latitude, season of conception and birth, maternal migration and ethnicity, as well as, nutritional, social or cultural factors, have been found to be related to the levels of vitamin D in mothers and the new-born (DeLuca et al. 2013, Kolevzon et al. 2007). Given that the measurement of vitamin D status during pregnancy or childhood, may provide indications for primary and secondary prevention interventions, the need for further studies in this field is identified.

Maternal infection and fever during pregnancy

Acetaminophen (or paracetamol) is one of the most commonly used analgesics worldwide. Many studies support that fever, particularly in the second trimester of gestation, is implicated in a subset of ASD cases (Hornig et al. 2018, Zerbo et al. 2013). Additionally, there is evidence that prenatal and early medicine therapy of the newborn is associated with autism (Grossi et al. 2016) and the exposure to acetaminophen during pregnancy has an increased risk for neurodevelopmental disorders and symptoms (Masarwa et al. 2018).

In one of the reviewed studies, Avella-Garcia et al. (2016) examined whether acetaminophen use in pregnancy was associated with attention function and autism spectrum symptoms at 1 and 5 years of age, by using data from the INfancia Medio Ambiente (INMA) project. This birth cohort study included participants from four different regions of Spain during different time periods: Asturias (2004–2007), Gipuzkoa (2006–2008), Sabadell (2004–2007) and Valencia (2004–2005). Data were collected prospectively by interviewing the expectant mothers twice, at weeks 12 and 32 of pregnancy, using standardized questionnaires. Exposure information was obtained by asking the question: ‘Have you taken any medication (sporadically or continuously) for 1 month before becoming pregnant or during this pregnancy?’ If the answer was positive, the name of the medication, dose, duration, gestational age at use and the indication as reported by the mother were enquired using open questions. At week 32, mothers were asked about use of medication after week 12. Neuropsychological development was assessed at a mean child age of 14.84 (SD = 2.69) months using the Bayley Scales of Infant Development (BSID). Children were tested again at a mean age of 4.8 (SD = 0.62) years with a battery of tests: McCarthy Scales of Children’s Abilities (MCSA), California Preschool Social Competence Scale (CPSCS), Childhood Autism Spectrum Test (CAST) and Conner’s Kiddie Continuous Performance Test (K-CPT), a computerized test that evaluates attention function, reaction time, accuracy and impulse control. Overall, 43% of children evaluated at age 1 (n = 2195) and 41% of those assessed at age 5 (n = 2001), were exposed to acetaminophen up to gestational week 32. Acetaminophen exposure was related to child hyperactivity/impulsivity at the age of 5 years, as well as, low attention function development. More autism spectrum symptoms only in boys may be due to possible gender differences in the metabolism of acetaminophen.

Acetaminophen has been shown to have endocrine disrupting properties and may interrupt maternal hormone signalling that regulates normal brain development (Tovo-Rodrigues et al. 2018). Furthermore, it may alter immune processes increasing the risk of autism spectrum disorder in susceptible individuals (Bauer and Kriebel 2013). However, although a causal link between exposure to acetaminophen and neurodevelopmental disorders cannot be established, careful inspection of current health policies is needed, due to the wide use of acetaminophen as a safe non-prescription medication (Masarwa et al. 2018). Therefore, the use of antipyretics and the frequency of exposure during pregnancy or in young children may be reserved for more severe fevers (Tovo-Rodrigues et al. 2018). Future studies should assess the risk/benefit relationship of use in pregnant women and infants undergoing fever or pain-related distress and evaluate the mechanisms through which acetaminophen could produce neurotoxicity.

Air pollution and pregnancy complications

In the study conducted by Grossi et al. (2016) solvents/paints occupational exposure during pregnancy and pregnancy complications emerged as important ASD associating factors. Mothers of ASD and typically developing children, originated in Lombardy (n = 37), central and southern Italian regions (n = 5). Participants from other European countries, also participated in the study Romania, Albania, Spain) (n = 3) took part in this study. A number of mothers, who worked in factories, closely to potentially toxic agents, reported leaving the place of work after discovering the pregnancy. Furthermore, the frequency of exposure to solvents or paints and pregnancy complications such as placental diseases, preeclampsia, or infections were significantly higher in the ASD group.

Prenatal and postnatal exposures to heavy metals have been involved in the pathogenesis of ASD (Goodrich et al. 2018). However, the role of heavy metals has not been fully defined. There are studies investigating the associations between ASD risk and exposure to trace elements (i. e. As, Zn, Pb, Hg, Cu), but they have been largely limited by indirect and cross-sectional methods of exposure measurement (Goodrich et al. 2018). Children are more susceptible than adults to environmental contaminants. They possess immature detoxification mechanisms and their heightened vulnerability is related to greater surface area relative to mass, nutritional aspects (they drink and eat more per unit of body weight) and behavioral patterns (mouth-touching behaviors) (Goodrich et al. 2018). All these hypotheses suggest the possibility of defects in the processes of metabolizing and eliminating heavy metals in children with ASD, contributing to the development of symptoms. Despite these results, further high-quality epidemiological studies on environmental toxicants and ASD, are warranted.

Labor risk factors

Mode of delivery, low birth weight and perinatal complications

Over the past decades, the prevalence of ASD children has been changed by factors such as birth weight, low gestational age, perinatal complications and the delivery type. Present evidence shows that there has been considerable increase in the rates of caesarean sections (CS), both in industrialized countries and in urban and rural areas of the developing world. Several studies have examined the possible relationship between caesarean and/or induced labor and ASDs, although the findings are conflicting (Gialloreti et al. 2014).

Six out of the 9 reviewed studies, investigated the role of these perinatal factors associated with ASD risk (Brayette et al. 2019, Cassimos et al. 2016, Grossi et al. 2016, Perales-Marín et al. 2018, Say et al. 2016, Ugur et al. 2019). In all of them, a significant difference was found for low gestational age, cesarean delivery and perinatal complications. In 3 studies (Brayette et al. 2019, Cassimos et al. 2016, Say et al. 2016), conducted in France, Greece and Turkey, respectively, ASD sample cases were more likely to have shorter gestation age and lower birth weight than controls. Moreover, incomplete gestation was found to have negative impact on both verbal and nonverbal cognitive abilities in children with ASD. In the retrospective study which was conducted in France (Brayette et al. 2019), the results showed that an incomplete birth (MLPT or ET, 37–38 WGA) has a negative impact on both verbal and nonverbal cognitive abilities, in children with neurodevelopmental vulnerability. Elevated prevalence of ASD is admitted for very preterm birth (25–31 WGA), suggesting that the critical period for atypical brain development underlying autism symptoms occurs before the MLPT period (Agrawal et al. 2018). In conclusion, incomplete gestation has an impact on both verbal and nonverbal cognitive abilities in children with neurodevelopmental vulnerability, without changing the ASD symptomatology. Therefore, the last weeks of gestation are very important for brain development (Brayette et al. 2019).

Postnatal factors

Maternal stress and breastfeeding

Several postnatal factors play a crucial role in ASD susceptibility and pathogenesis. After delivery, maternal stress in particular during the first 6 months of the child’s life, can create important alterations in the early affective mother-child interaction, conditioning emotional-behavioral responses of the offspring. Additionally, not breastfeeding or short duration of breastfeeding and early antibiotic treatment of the newborn may be related to ASD (Grossi et al. 2016). It has been demonstrated that breastfeeding through the transference of oxytocin in breast milk, is shown to contribute to social recognition, social bonding and neurodevelopment in the infant (Krol et al. 2015). In addition, there is a unanimous consensus about the fact that vaginal delivery and breastfeeding are essential in providing the appropriate starting bacterial substrate for the development of a physiological gut flora (Yasmin et al. 2017). The possibility that ASD is associated with the imperfect development of gut flora is supported by the frequent coexistence of gastrointestinal symptoms in ASD children and the appearance of the disease after an incidental antimicrobic therapy (Grossi et al. 2016).

Implications

The present review aims at generating a concise summary of novel dominant environmental determinants of autism in Southern Europe, through a systematic synopsis of recent empirical data. While genetics is now a well-established risk factor, several data support the contribution of multiple environmental factors. Consequently, the results of such a review could have implications for recurrence risk estimation and genetic counselling. They could, also, have significant impact on the detection of the interactions among the diverse and multiple risks and protective factors, associated with ASD. However, given that the impact of several of these ASD risk factors is only partially clarified, the clinical recommendations should be pondered in the perspective of the precautionary principle. Some general recommendations which are related to prenatal and antenatal care for each pregnancy include no smoking, no alcohol consumption and an appropriate diet during pregnancy. In addition, obstetricians and paediatricians should pay attention to environmental conditions, which are recognized as potential risk factors for ASD, such as advanced parental age, a sibling with ASD, prematurity, maternal stress, as well as early-life antibiotic use. In all these cases, close monitoring aiming at minimizing the effects of risk factors and maximizing the impact of protective factors is warranted.

Limitations

This review includes only original studies from Southern Europe published from 2010 up to 2020, which investigate potential environmental factors associated with ASD. It ascertains a recent, but decreased, research interest in Southern Europe around this topic compared to the rest of Europe as reflected by the small number of the papers included in the current review. In order to provide reliable results with respect to certain environmental factors associating with ASD in Southern Europe, more relevant studies are urgently needed. As research in environmental risk for ASD is constantly growing, more concept suggestions might have resulted in more data. Moreover, many of the reviewed studies investigate single factors rather than a combination of multiple environmental agent associated with increased risk of ASD symptoms.

Conclusion

Given the fact that ASD is a multifactorial disorder, much scientific evidence suggests that beyond genetics, the contribution of environmental factors and their interaction with genes, is also important (Hadjkacem et al. 2016). Identification of the most critical windows of developmental vulnerability is paramount to understanding when and under what circumstances, a child is at elevated risk for ASD. The current review suggests that ASD in Southern Europe is associated with a variety of prenatal, perinatal and postnatal environmental factors, including high parental age, stressful early life events, maternal infection during pregnancy, pregnancy or perinatal complications, delivery mode and breastfeeding problems in the offspring. Cesarean mode of delivery has been identified as an independent factor for ASD risk in combination with absence of breastfeeding and early antibiotic use. The underlying mechanisms, however, for these associations are incompletely understood. On the contrary, levels of vitamin D₃ in mothers_, nutritional factors and occupational exposure to solvents/paints during pregnancy, were found to have a less strong (but significant) association with ASD risk. It is worth pointing out that in Southern Europe the environmental risk factors for ASD do not seem to follow a specific pattern, probably due to potential regional or local factors within this geographical area (i.e. hours of sunlight affecting vitamin D, nutritional patterns and behaviours, etc.). Although the reviewed findings largely agree with findings from similar studies in Northern Europe (Hultman et al. 2002, Haglund and Källén 2011, Persson et al. 2020, Rai et al. 2012, Sandin et al. 2016), inconsistent results have emerged for vitamin D₃ deficiency in mothers in these areas and, also, exposure to polluted air during pregnancy, both associating with higher ASD prevalence (Gong et al. 2017, Guxens et al. 2016, Lee et al. 2021, Mazahery et al. 2016, Ritz et al. 2018, Schmidt et al. 2019). However, as stressed above, regional or global inconsistencies may be understood as deriving from differences among countries in degrees of latitude, sunshine duration and exposure, sources and level of air pollution or other cultural and dietary variations. Moreover, methodological differences among studies must be seriously considered when comparing evidence on environmental factors and factor interactions contributing to ASD aetiology.

To conclude, prenatal, perinatal and postnatal factors should be considered as events which interact in combination with other co-factors to characterize ASD. Consequently, future studies should move to models which take into account the dynamic relationship between genetics and environment by using large, population-based birth cohorts and to allow for precise assessments of potential confounders in order to further elucidate the role and the interplay of specific environmental factors associated with the condition. In addition, future research in the regions of Southern Europe would benefit if they would take into consideration certain basic criteria, such as geographical, nutritional, socioeconomic or cultural factors which probably may differentiate the association of environmental risk factors with ASD. Determining the contribution of these risk factors may improve detection, earlier treatment and better prevention of the disorder.

Funding Statement

The authors received no financial support for the research, authorship, and/or publication of this article.

Disclosure statement

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.