Table 1. Indoor air parameters...

Table 1.

Indoor air parameters associated with health outcomes in schools

Feature	Health outcomes	Target value/guidelines
Ventilation rate	Increased ventilation rates were associated with decreased illness absence (Mendell et al., 2013)	According to EN 16798–1 (European Committee for Standardization, 2019), the total ventilation rate must never be lower than 4 l/s per person. The Finnish standard specifies the minimum ventilation rate per person 6 l/s in schools (Finnish Ministry of The Environment, 2017)
	Higher ventilation rates, even up to 15 l/s per person or higher, in classrooms were consistently associated with decreased illness absence (Mendell et al., 2013)
	A small increase in reported eye symptoms at the higher ventilation rates were found but no association with other self-reported symptoms (Petersen et al., 2016)
	Increased ventilation rate was significantly associated with decreased odds of suffering from eye and skin disorders (Baloch et al., 2020)
CO₂	A CO₂ concentration above 1,000 ppm in classrooms was associated with considerable increases in dry cough and rhinitis (Simoni et al., 2010)	A European standard specifies the maximum CO₂ concentration in a classroom with moderate indoor air quality of 600–1000 ppm (CEN European Committee for Standardization, 2004). According to a Finnish standard (Finnish Ministry of The Environment, 2017), the maximum CO₂ concentration in a new building is 800 ppm above the level of CO₂ outdoors
	CO₂ concentration was negatively correlated with general symptoms and mucosal symptoms (Zhang et al., 2011)
	Higher level of CO₂ was associated with itchiness and nasal congestion (Fernández-Agüera et al., 2019)
	Allergies, nose irritation and fatigue were significantly positively correlated to the levels of CO₂ concentrations (Dorizas et al., 2015)
	Ocular, nasal and throat symptoms, breathlessness, headache and tiredness were significantly more common at higher CO₂ levels (Norbäck and Nordström, 2008)
	Increased classroom CO₂ had a relatively small, but significant, effect on wheezing but not on other health problems, e.g. headaches (Kim et al., 2011)
	Current asthma, asthma attacks and asthma medication were positively associated with CO₂ (Mi et al., 2006)
	Teachers’ neuro-physiologic (i.e. headache, fatigue, difficulty concentrating) symptoms significantly increased for every 100 ppm increase in maximum classroom CO₂ concentrations and were nonsignificantly increased in classrooms with above-median proportions of CO₂ concentrations greater than 1,000 ppm (Muscatiello et al., 2015)
	Higher level of CO₂ was associated with an increased incidence of mucosal symptoms and general symptoms (Zhang et al., 2014)
	CO₂ values higher than 2,100 ppm were associated with exercise-induced wheezing and night cough among pupils (Fraga et al., 2008)
	Higher levels of CO₂ in schools were connected with a higher risk of experiencing upper, lower airways and systemic outcomes (Baloch et al., 2020)
Air pressure	Positive air pressure (5–7 Pa) decreased nose irritation, dryness of facial skin and tiredness in a school, where occupants’ symptoms were suspected to be related to the impurities leaked indoors through the building envelope (Vornanen-Winqvist et al., 2018a, 2018b)	According to the National Building Code of Finland, in a new building, positive air pressure should not cause long-term moisture load into the structures and negative air pressure should not allow harmful contaminant to enter into indoor air (Finnish Ministry of The Environment, 2017)
Air humidity	Modest increase in respiratory symptoms over both at low (<30%) and high RH (>50%) among teachers (Angelon-Gaetz et al., 2016)
	Current asthma, asthma attacks and asthma medication were positively associated with indoor relative air humidity (Mi et al., 2006)
	Relative humidity was associated with increased odds of suffering from lower airways and systemic disorders among schoolchildren (Baloch et al., 2020)
Room temperature	Increased room temperature was related to nasal and throat symptoms, headache and tiredness (Norbäck and Nordström, 2008)	In Finland, target value for room temperature is 20°C– 23°C during heating season (Finnish Ministry of the Environment, 2012)
	Room temperatures above 22 °C increased mucosal irritation and general symptoms such as headache and tiredness may occur in temperate climates (Zhang et al., 2011)
NO₂	An increase in indoor NO₂ was associated with current asthma and asthma medication. Outdoor NO₂ was associated with current asthma (Mi et al., 2006)	The WHO guideline value is 200 μg/m3 for 1-h NO₂ concentration (World Health Organization, 2010)
	NO₂ might be related to mucosal symptoms, and skin symptoms (Zhang et al., 2011)
	Pupils reported more wheezing at increased outdoor level of NO₂ (Kim et al., 2011)
SO₂	SO₂ might be related to general symptoms, mucosal symptoms and skin symptoms (Zhang et al., 2011)	The 24-h average WHO guideline value for SO₂ is 20 μg/m3 (World Health Organization, 2006)
	Both indoor and outdoor SO₂ were positively associated with prevalence of school-related symptoms (Zhang et al., 2014)
O₃	Indoor O₃ was positively associated with prevalence of skin symptoms (Zhang et al., 2014)	The daily maximum 8-h mean of WHO guideline value for O₃ is 100 μg/m3 (World Health Organization, 2006)
	Elevated levels of ozone were significantly related to a variety of symptoms (Baloch et al., 2020)
VOC	Pupils exposed to higher total VOC concentrations (the median concentration of 140.3 μg/m³) had a twofold increased risk of having asthma-related symptoms (Madureira et al., 2015)	WHO recommendation is 100 µg/m³ (30 min) (World Health Organization, 2010). Finnish standard recommends the action level of 400 µg/m³ (Valvira, 2016)
	Increased number of VOCs was connected to a higher risk for upper and lower airways and systemic disorders (Baloch et al., 2020)
Formaldehyde	Indoor exposure to formaldehyde was related to asthma-like symptoms among pupils (Madureira et al., 2015)	WHO and Finnish standard specify the maximum short-term (30 min) guideline of 0.1 mg/m3 (Finnish Ministry of the Environment, 2012; World Health Organization, 2010)
	A high prevalence of rhinoconjunctivitis in the past year was found in children using classrooms with high levels of formaldehyde (Annesi-Maesano et al., 2012)
Particulate matters (PM_2.5 and PM₁₀): contaminants, including viruses, bacteria, allergens, fungi and traffic emissions	Both indoor and outdoor PM₁₀ was associated with increased incidence and decreased remission of most types of SBS symptoms such as facial and hand rash or itching; eye irritation; nasal catarrh and obstruction; dryness in the throat; sore throat; irritating cough; headache (Zhang et al., 2014)	The WHO health based standard for PM₁₀ is a maximum of 50 µg/m³ as a 24-h mean value (World Health Organization, 2006). According to the Finnish standard, the maximum design value is also 50 µg/m3 for PM₁₀ and 25 µg/m3 for PM_2,5 (Finnish Ministry of the Environment, 2012)
	The indoor median concentration of PM2.5 and PM10 that exceed the 25 μg/m³ and 50 μg/m³ levels of WHO air quality guidelines for a sampling period of 24 h increased the odds of asthma-like symptoms among pupils (Madureira et al., 2015)
	High fungal concentrations were associated with respiratory symptoms of children in concrete/brick schools but not in wooden schools (Meklin et al., 2002)
	Certain moulds were associated with the respiratory symptoms of children (Simoni et al., 2011)

Feature	Health outcomes	Target value/guidelines
Ventilation rate	Increased ventilation rates were associated with decreased illness absence (Mendell et al., 2013)	According to EN 16798–1 (European Committee for Standardization, 2019), the total ventilation rate must never be lower than 4 l/s per person. The Finnish standard specifies the minimum ventilation rate per person 6 l/s in schools (Finnish Ministry of The Environment, 2017)
	Higher ventilation rates, even up to 15 l/s per person or higher, in classrooms were consistently associated with decreased illness absence (Mendell et al., 2013)
	A small increase in reported eye symptoms at the higher ventilation rates were found but no association with other self-reported symptoms (Petersen et al., 2016)
	Increased ventilation rate was significantly associated with decreased odds of suffering from eye and skin disorders (Baloch et al., 2020)
CO₂	A CO₂ concentration above 1,000 ppm in classrooms was associated with considerable increases in dry cough and rhinitis (Simoni et al., 2010)	A European standard specifies the maximum CO₂ concentration in a classroom with moderate indoor air quality of 600–1000 ppm (CEN European Committee for Standardization, 2004). According to a Finnish standard (Finnish Ministry of The Environment, 2017), the maximum CO₂ concentration in a new building is 800 ppm above the level of CO₂ outdoors
	CO₂ concentration was negatively correlated with general symptoms and mucosal symptoms (Zhang et al., 2011)
	Higher level of CO₂ was associated with itchiness and nasal congestion (Fernández-Agüera et al., 2019)
	Allergies, nose irritation and fatigue were significantly positively correlated to the levels of CO₂ concentrations (Dorizas et al., 2015)
	Ocular, nasal and throat symptoms, breathlessness, headache and tiredness were significantly more common at higher CO₂ levels (Norbäck and Nordström, 2008)
	Increased classroom CO₂ had a relatively small, but significant, effect on wheezing but not on other health problems, e.g. headaches (Kim et al., 2011)
	Current asthma, asthma attacks and asthma medication were positively associated with CO₂ (Mi et al., 2006)
	Teachers’ neuro-physiologic (i.e. headache, fatigue, difficulty concentrating) symptoms significantly increased for every 100 ppm increase in maximum classroom CO₂ concentrations and were nonsignificantly increased in classrooms with above-median proportions of CO₂ concentrations greater than 1,000 ppm (Muscatiello et al., 2015)
	Higher level of CO₂ was associated with an increased incidence of mucosal symptoms and general symptoms (Zhang et al., 2014)
	CO₂ values higher than 2,100 ppm were associated with exercise-induced wheezing and night cough among pupils (Fraga et al., 2008)
	Higher levels of CO₂ in schools were connected with a higher risk of experiencing upper, lower airways and systemic outcomes (Baloch et al., 2020)
Air pressure	Positive air pressure (5–7 Pa) decreased nose irritation, dryness of facial skin and tiredness in a school, where occupants’ symptoms were suspected to be related to the impurities leaked indoors through the building envelope (Vornanen-Winqvist et al., 2018a, 2018b)	According to the National Building Code of Finland, in a new building, positive air pressure should not cause long-term moisture load into the structures and negative air pressure should not allow harmful contaminant to enter into indoor air (Finnish Ministry of The Environment, 2017)
Air humidity	Modest increase in respiratory symptoms over both at low (<30%) and high RH (>50%) among teachers (Angelon-Gaetz et al., 2016)
	Current asthma, asthma attacks and asthma medication were positively associated with indoor relative air humidity (Mi et al., 2006)
	Relative humidity was associated with increased odds of suffering from lower airways and systemic disorders among schoolchildren (Baloch et al., 2020)
Room temperature	Increased room temperature was related to nasal and throat symptoms, headache and tiredness (Norbäck and Nordström, 2008)	In Finland, target value for room temperature is 20°C– 23°C during heating season (Finnish Ministry of the Environment, 2012)
	Room temperatures above 22 °C increased mucosal irritation and general symptoms such as headache and tiredness may occur in temperate climates (Zhang et al., 2011)
NO₂	An increase in indoor NO₂ was associated with current asthma and asthma medication. Outdoor NO₂ was associated with current asthma (Mi et al., 2006)	The WHO guideline value is 200 μg/m3 for 1-h NO₂ concentration (World Health Organization, 2010)
	NO₂ might be related to mucosal symptoms, and skin symptoms (Zhang et al., 2011)
	Pupils reported more wheezing at increased outdoor level of NO₂ (Kim et al., 2011)
SO₂	SO₂ might be related to general symptoms, mucosal symptoms and skin symptoms (Zhang et al., 2011)	The 24-h average WHO guideline value for SO₂ is 20 μg/m3 (World Health Organization, 2006)
	Both indoor and outdoor SO₂ were positively associated with prevalence of school-related symptoms (Zhang et al., 2014)
O₃	Indoor O₃ was positively associated with prevalence of skin symptoms (Zhang et al., 2014)	The daily maximum 8-h mean of WHO guideline value for O₃ is 100 μg/m3 (World Health Organization, 2006)
	Elevated levels of ozone were significantly related to a variety of symptoms (Baloch et al., 2020)
VOC	Pupils exposed to higher total VOC concentrations (the median concentration of 140.3 μg/m³) had a twofold increased risk of having asthma-related symptoms (Madureira et al., 2015)	WHO recommendation is 100 µg/m³ (30 min) (World Health Organization, 2010). Finnish standard recommends the action level of 400 µg/m³ (Valvira, 2016)
	Increased number of VOCs was connected to a higher risk for upper and lower airways and systemic disorders (Baloch et al., 2020)
Formaldehyde	Indoor exposure to formaldehyde was related to asthma-like symptoms among pupils (Madureira et al., 2015)	WHO and Finnish standard specify the maximum short-term (30 min) guideline of 0.1 mg/m3 (Finnish Ministry of the Environment, 2012; World Health Organization, 2010)
	A high prevalence of rhinoconjunctivitis in the past year was found in children using classrooms with high levels of formaldehyde (Annesi-Maesano et al., 2012)
Particulate matters (PM_2.5 and PM₁₀): contaminants, including viruses, bacteria, allergens, fungi and traffic emissions	Both indoor and outdoor PM₁₀ was associated with increased incidence and decreased remission of most types of SBS symptoms such as facial and hand rash or itching; eye irritation; nasal catarrh and obstruction; dryness in the throat; sore throat; irritating cough; headache (Zhang et al., 2014)	The WHO health based standard for PM₁₀ is a maximum of 50 µg/m³ as a 24-h mean value (World Health Organization, 2006). According to the Finnish standard, the maximum design value is also 50 µg/m3 for PM₁₀ and 25 µg/m3 for PM_2,5 (Finnish Ministry of the Environment, 2012)
	The indoor median concentration of PM2.5 and PM10 that exceed the 25 μg/m³ and 50 μg/m³ levels of WHO air quality guidelines for a sampling period of 24 h increased the odds of asthma-like symptoms among pupils (Madureira et al., 2015)
	High fungal concentrations were associated with respiratory symptoms of children in concrete/brick schools but not in wooden schools (Meklin et al., 2002)
	Certain moulds were associated with the respiratory symptoms of children (Simoni et al., 2011)

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