In Italy (Table 1), it became clear in the early nineties that the fuel for cooking or for heating and that kind of heating are associated with increased prevalence rates of respiratory symptoms or impaired lung function. In PD males, significantly increased odds ratios (ORs) were found for: cough and dyspnea with bottled gas for cooking; cough and phlegm with stove for heating; cough with natural gas stove and fan or stove powered by other fuels.23 In females, significantly increased ORs were found only for dyspnea with bottled gas for cooking, stove for heating, natural gas stove, stove or fan powered by other fuels.23 In PI males, significantly increased ORs were shown for: cough and phlegm associated with bottled gas for cooking in males; wheeze and shortness of breath with wheeze associated with the use of a stove or forced-air circulation in females.24 In 8–19 years PD non-smokers, significantly higher prevalence rates of wheeze, dyspnea, diagnosis of asthma were found in subjects of both sexes using bottled gas for cooking and exposed to passive smoking.22

The importance of secondhand smoke (SHS) exposure as a risk factor for respiratory diseases was also pointed out. Higher occurrence of cough and phlegm in young non-smokers and of acute respiratory symptoms in the elderly exposed to SHS were shown in PD and PI studies.25,28 In 867 PD non-smoking women, SHS was a significant risk factor for asthma.26 In 2195 non-smoking women, living in PI, PD, Rome and Viterbo, SHS exposure both to husband/partner and at work was a significant risk factor for dyspnea, cough/phlegm and rhino-conjunctivitis, any shortness of breath at rest, wheeze and attacks of shortness of breath with wheeze, asthma diagnosis/symptoms, obstructive lung diseases (diagnosis of asthma/chronic bronchitis/emphysema).14 In 2150 PA schoolchildren, SHS was responsible for 18.1% of current asthma and 18.8% of rhino-conjunctivitis.16 Passive smoking exposure during pregnancy was related to a higher risk of uncontrolled asthma in a recent cohort study on PA asthmatic children.21

In the SIDRIA-2 study, carried out in 2002 in 12 areas of northern, central and southern Italy, mold exposure in the first year of life was positively associated with wheezing, asthma and rhino-conjunctivitis in children and adolescents, and with cough/phlegm only among children. Mould exposure in the previous year was a significant risk factor for wheeze, among children, and wheeze was most strongly related to early and current mould/dampness exposure; the same occurred in adolescents for rhino-conjunctivitis.29,30 In 2150 PA children, mould/dampness exposure was responsible of about 7% of current asthma and rhino-conjunctivitis cases.16

In the SIDRIA-2 study, dog exposure in the first year of life increased the risk for wheezing in children and for cough/phlegm in both children and adolescents. Lifetime dog exposure was significantly related to cough/phlegm in adolescents. Finally, cat exposure in the first year of life was significantly associated with current wheezing, asthma and rhino-conjunctivitis in children.15,29 In PA children, cat and dog exposures were responsible of 7.2% and 5.3% of current asthma cases, and of 1.4% and 1.0% of rhino-conjunctivitis cases, respectively.16

In Italy (Table 1), the direct measurements of particulate matter < 2.5 µm (PM2.5) and nitrogen dioxide (NO2) were carried out in 140 PD households during winter and summer. The concentration of monitored pollutants was larger in winter than in summer. Significantly higher indoor NO2 levels were found in the houses with gas-furnace heating and/or with gas water heater located inside the home. PM2.5 was significantly higher in smokers’ homes, and significantly related to the number of cigarettes smoked.33 Only in winter, there were significant associations of bronchitic/asthmatic symptoms with NO2 and PM2.5 values above the median. In summer, this association emerged for PM2.5 only in non-smokers.27 Moreover, in PD and PI adults, NO2 and PM2.5 exposures were associated with acute respiratory symptoms and mild lung function impairment.12 In an elderly sample, the occurrence of acute respiratory symptoms was consistently higher in relation to the high respirable suspended PM (RSP) exposure, compared to low exposure.28

In the GERIE study, for the first time, the relationships between indoor air quality, comfort parameters and respiratory morbidity among European elderly people living in nursing homes were investigated. The following associations were found: particulate matter <0.1 µm (PM0.1) with forced expiratory volume in one second (FEV1)/ forced vitality capacity (FVC) <70% and wheeze; PM10 and CO2 with breathlessness and cough; NO2 with FEV1/FVC<70%, breathlessness and cough; formaldehyde with COPD and exhaled carbon monoxide. Pollutant effects were more pronounced in the case of poor ventilation.18

In the HESE study, schoolchildren exposed to CO2 levels >1000 ppm showed a higher risk for dry cough and rhinitis; nasal patency was significantly lower in those exposed to PM10 >50 µg·m − 3.31

PA adolescents, exposed to the highest indoor NO2 concentrations, had increased frequency of current asthma, wheezing in the last 12 months, chronic phlegm, and rhino-conjunctivitis.17

In the RESPIRA project, assessment of respiratory health, determination of PM elemental composition and of selective markers of smoking were performed in 73 houses of adolescents living in Sicily (Gela, Niscemi, Mazzarino and Butera). When corrected for confounding factors, PM2.5, Cadmium (Cd), Cerium (Ce), Lanthanum (La) and Thallium (Tl) were associated with increased probability of having respiratory symptoms.20 Further, Dermatophagoides pteronyssinus (Der p 1) was positively correlated to both high Der p 1-specific IgE and Fractional exhaled nitric oxide (FeNO) in children with last 12 months wheeze.19

In the HESE study, an elevated concentration of moulds (≥300 cfu/m3) in the classroom was associated with higher risk of dry cough at night, persistent cough and rhinitis in the last 12 months. Aspergillus/Penicillium DNA was associated with wheeze, and Aspergillus versicolor DNA with wheeze, rhinitis and cough; there were inverse associations of Aspergillus versicolor DNA with FVC and Streptomyces DNA with both FEV1 and FVC.32

In Mexico (Table 2), 9–12 year-old children spend 85% of their time indoors.34 Airborne pollen, indoor and outdoor pollutants may influence sick building syndrome (SBS).35 In a study carried out in Monterrey (Nuevo Leon) 91% of subjects reported at least 1 SBS symptom, and fatigue was the most frequent (74.6%). Quercus and Fraxinus pollen were related with headaches, while humidity and NOx were associated with eye and nasal symptoms.35

In 5210 elementary schoolchildren of the El Paso Independent School District, ant and spider pest problems within the past year, pet dogs, fireplace heat, central air conditioning, humidifier use, and cooking with gas stoves were positively associated with both allergy and asthma prevalence (ever and current). Lung function decreased among children who lived in homes with reported cockroach pest problem in the past year without concurrent use of pesticides.36

In 841 non-smoking ≥35 year-old women in the village of Solis (near Mexico City), compared with those cooking with gas, current use of a stove burning biomass fuel was associated with increased reporting of phlegm and reduced FEV1/FVC.37 In >35 year-old females in a suburban area of Mexico City, spirometry and oxygen saturation values were significantly lower with higher biomass exposure levels. Cough, phlegm, wheezing and dyspnea were significantly more frequent in women exposed to biomass.38

In 8–17 year-old schoolchildren from Mexico City and its metropolitan area (n = 3187), passive smoking was associated with a higher frequency of self-reported respiratory symptoms and respiratory infections. Levels of FEV1 and FVC in children exposed to passive smoking were reduced: such values decreased with increasing number of smokers at home and higher outdoor ozone levels.39 A study which involved 536 mother-child couples showed significant associations of higher prenatal and early life PM2.5 exposure with higher cumulative risk ratios of ever wheeze and current wheeze in the past year only among 6–8 year-old children born to mothers exposed to SHS in pregnancy.40

A significant association between dust and sensitivity to dust mite was found in homes of sensitized children in Hermosillo (Sonora).41 In 42 adult asthma patients in Mexico City, mould burden was negatively associated with parameters of asthma control only in males.42

Identification and quantification of persistent indoor organic pollutants were carried out in winter within three bus lines of Leon (Guanajuato) in order to determine their possible origin, the differences in the levels of contamination between routes, and the potential risk to the health of the users. Xylenes were the most representative organic pollutants with the potential to generate a higher risk to bus-users’ health (hazard quotient: 6.4–9.8), without dismissing the potential danger of other pollutants.43

In Brazil (Table 2), wood smoke is a major source of indoor air pollution, mainly among non-affluent people in rural areas.44 In Bahia state, childhood exposure to indoor wood smoke was reported by 84% adult rural-born asthmatics vs only 27% urban-born subjects;45 1.7% of asthmatic urban dwellers reported current use of wood stoves for cooking.46 In the north and northeast region, residential wood burning was significantly associated with higher risk of bronchitis/asthma only in women.44 The PURE study, a multinational 10 year cohort study on subjects from 22 urban communities and 42 rural areas, showed that Brazilians using solid fuel for cooking had higher cardiorespiratory mortality rate, compared to users of electric or gas stoves.47 In 1402 subjects living in Joao Camara, significantly increased ORs for cough, wheezing and dyspnea emerged in adults exposed to indoor stove and outside bonfire, compared to liquefied petroleum gas. Both non-smokers exposed to biomass and smokers exposed to gas had decreased %predicted-FEV1 and FEV1/FVC, as compared to non-smokers exposed to gas.48

Policies increasing access to clean fuel strongly reduced the utilization of wood fire during the last five decades.49 However, the interruption of the exposure does not avoid completely its late effects on respiratory health: in fact, previous exposure to indoor wood smoke predicts later uncontrolled asthma symptoms and lung function impairment in subjects no longer exposed, in comparison to never exposed ones.46

Tobacco-free policy contributed to the decreasing prevalence of smoking during the last few decades, but second-hand smoking remains an important source of indoor air pollution.50 It is associated with central and peripheral airway obstruction in children and adolescents.51 In adults, passive smoking is associated with impaired mucociliary clearance: exposure load correlates with worse hemodynamic parameters, reduced lung function and autonomic nervous system dysfunction.52 The probability of wheezing increased with the number of cigarettes smoked indoors in 105 children (6–23 months of age).53

In 375 Belo Horizonte adolescents, asthma symptoms were related to contact with animals during the first year of life and over 20 cigarettes per day smoked by parents or other household members.54 In 878 9–12 year-old children in Passo Fundo (Rio Grande do Sul), having an indoor cat during the first year of life was associated with asthma.55

PM is another source of indoor air pollution in large urban centers. In 2018, a strong correlation between indoor and outdoor concentrations of PM10 and PM2.5 in a nursing home in São Paulo City over thirty days of sequential sampling was observed.56 During the sampling period, indoor PM concentration exceeded the 2005 WHO guidelines.56 In the same year, high indoor concentrations of fine PM were observed in most studied residences in São Paulo metropolitan area, with indoor PM2.5 exceeding WHO recommendations in 43% of the residences; the highest values occurred in houses near heavy traffic areas during non-precipitating days.57 However, recent studies relating indoor concentration of PM and respiratory health are lacking.

In Vietnam (Table 2), > 64% people live in rural areas with less accessibility to cleaner fuels. Energy Information Administration reported that about 25% of the total Vietnam's energy consumption was from solid biomass fuels and wastes: nearly 60% of biomass was consumed by households, mainly in rural areas.58

SHS is an important source of HAP: male smoker prevalence is very high (45.3%).

Despite the public places’ bans, SHS prevalence among adults ≥15 years remained high (89.1% in Bars/Cafes/Tea shops and 80.7% in restaurants, 37.9% in government offices, and 30.9% in universities).59

House ventilation is poor, especially in crowded city. People live very close to roadways and busy streets, which leads to increase PM and NO2 inside the houses. Recent studies, measuring outdoor and indoor air quality in Hanoi, were focused on particle number and particle mass concentrations, which showed the influence of traffic and construction activities in both outdoor and indoor environment.60,61

In South-East Asia, high temperature and humidity are an ideal habitat for mites and cockroaches.62,63 In Vietnam, the humidity and dampness are high, especially in Red River and Mekong delta.

Thus, moulds, bacteria, insects and mites grow rapidly and are linked to asthma, chronic rhinosinusitis, hypersensitivity pneumonitis and exacerbations of underlying respiratory diseases.64

In urban and rural environments, there are many allergens from house dust mites, cockroaches, and mouse infestations, contributing to respiratory diseases such as wheeze, asthma attacks, asthma, and allergic rhinitis, as emerged in 864 adults living in northern Vietnam.62

Many domestic behaviors and practices are associated with increased HAP: incense burning, poor ventilation and keeping moto-bikes in small houses, high concentrations of CO2 and volatile organic compounds (VOCs).

To reduce HAP in rural areas, use of cleaner biomass burning stoves, chimney for kitchen and gas to replace biomass fuels were incentivized. However, traditional way of cooking is still mostly used in remote areas.

In urban regions, better ventilation and use of air cleaners were encouraged, but the problem of residential overcrowding is very difficult to solve. Reducing the prevalence of smoking is a major goal, but it is time consuming.

The Projet Interuniversitaire Cible (PIC), between universities of Vietnam and Belgium from the year 2014 to 2023 aiming at reducing HAP, is being carried out with some preliminary positive results.65

In a study conducted among 900 ≥ 30 year-old non-smoking women, from 45 rural villages of Tiruvallur district (Tamil Nadu, India) (Table 2), COPD prevalence was higher in biomass fuel users than in clean fuel users.66 The same association emerged for asthma in women within a nationwide large-scale cross-sectional survey on 156,316 subjects aged 20–49 years.67 In a survey on ≥35 year-old adults in 5 villages of Tiruvallur district, combined use of all three biomass fuel types (wood, dung and crop) and use of mosquito coil ≥5 days per week were associated with poor respiratory health.68 In 214 <5 year-old Indian children, cooking fuel other than liquid petroleum gas was found to be significant risk factors for acute LRI.69 From 2001 to 2011, use of cooking fuel was associated with total mortality in <5 years children from Nepal, with stronger associations evident for ≤28 days infants mortality.70 Associations were found also with acute respiratory tract infections (ARI).71

In a Nepalese population sample, prevalence of airflow obstruction was significantly higher in subject exposed to the domestic biomass burning, compared with those using liquid petroleum gas fuel.72 In 1200 ≥ 30 year-old adults in Mehrauli, a large urban community in South Delhi (India), SHS and biomass fuel use were found among the risk factors associated with COPD.73

In 150 clinically diagnosed 3–12 year-old asthmatics, damp environment, pets and passive smoking were significant risk factors for asthma.74

In a multicentric population study carried out in India, SHS exposure during childhood is an important risk factor for asthma and respiratory symptoms in non-smoking adults: a higher risk of having asthma emerged for exposures in childhood only or both in childhood and adulthood.75 In a Nepalese study, children asthma was associated with cigarette smoking by two or more family members and domestic use of smoky fuels.76 In a Nepalese study carried out on 6056 children, maternal smoking had a significant effect on children's ARI symptoms, mainly among those born to mothers who smoked more frequently.77 More recently, in a study carried out in Kathmandu, non-exclusive breastfeeding (with respect to exclusive one) increased the effect of household passive smoking on ARI.78

In Kyrgyzstan (Table 2), indoor pollution is a major risk factor among highlanders who use only biomass for cooking and heating, in poorly ventilated rooms. High level of indoor air pollution may cause respiratory symptoms among highlanders.79,80

In a study conducted in a highland and a lowland rural setting, COPD prevalence and indoor PM2.5 exposure was highest in the highlands, and were independently associated.81

The FRESH AIR research program found biomass use for heating and cooking by 71.2% and 52.0% of a sample of COPD patients, respectively;82 therefore, the health impact of reducing biomass exposure could be considerable. Among 99 adults and children living in rural communities, after implementing improved cookstoves/heaters, significant reductions of PM2.5 concentration (65%) and of daily respiratory symptoms and chest infections were found.83

The IPCRG is a clinically-led charitable organization for improving prevention, diagnosis and care of respiratory diseases in community settings around the world through research and education. Since 2011, in collaboration with multiple partners, IPCRG has been engaging with stakeholders in eight resource-limited settings in three continents to measure and to co-create evidence-informed programmes addressing exposure to poor indoor air and its impact on respiratory health. Where evidence about relevant interventions existed, implementation science methods were applied to test its relevance, impact and generalizability in different settings. Where it was lacking, knowledge gaps were identified and tacit colloquial and local evidence elicited to enrich understanding and to inform the development of new interventions.84

An IPCRG's FRESH AIR protocol was developed in Vietnam and in rural Uganda, contributing to an understanding that COPD may start early in life, and prevalence is highest in men and women aged 30–39, due to both high male smoking rates and also exposure to indoor biomass.85,86 RESPIRE, a Global Health Research Unit at the University of Edinburgh funded by the National Institute for Health Research (NIHR), aims to reduce the impact and number of deaths caused by respiratory diseases in Asia. A 2019 systematic review by RESPIRE found that the global prevalence of COPD among 30–79 year-old people was between 7.6%−10.3% depending on the definition used, and that smoking, biomass exposure and occupational exposure to dust or smoke were all substantial risk factors for COPD.87 RESPIRE and FRESH AIR researchers subsequently collaborated to review prevalence survey methodology for CRDs in LMICs to inform new surveys in Asia, finding that methods to map and measure COPD are more reliable than asthma.88

An adapted FRESH AIR protocol was applied in four countries with limited resources (Vietnam, Crete, Uganda and the Kyrgyz Republic), as part of a European Commission Horizon 2020 programme 2015–2019.89 It included:

• •

  Structured rapid assessment highlighting three key factors that needed to be addressed: 1. perceived CRD identity when systems and education have been primarily about respiratory infection leading to inaccurate attribution of symptoms; 2. beliefs about causes of CRD: 65% strongly agreed that tobacco smoking caused symptoms, but only 19% for household air pollution; and other perceived causes included witchcraft and a hot-cold disbalance and 3. norms and social structures for example “real men smoke”.90

• •

  Capacity building for health care providers (HCPs) to deliver Very Brief Advice (VBA) for quitting tobacco, pulmonary rehabilitation and community education about the harms of indoor air pollution from burning biomass fuels in the home.91

• •

  Use of culturally appropriate engagement strategies including the use of photography and film.

It was concluded that contextualizing the programmes is feasible, acceptable and effective, and increases their sustainability.91

An estimated 2.6–3 billion people rely on solid fuels for cooking or heating increasing their risk of harm to their respiratory health. The impact of financial austerity may increase this use.92 Therefore, accelerating access to cleaner solutions was a target of many studies from the perspective of health, environment, climate and gender equity: yet, the results tended to be disappointing. IPCRG coordinates the Global Health Respiratory Network (GHRN), established in 2018, focusing on the poorest communities embracing use of clean cookstoves and clean fuel.93 Two studies explored the barriers to uptake and found that funds, knowledge and beliefs about the innovation of solid fuel cookstoves and clean fuels were common barriers. External policy and incentives are also important for clean fuels.94 Meanwhile, “disconnects” for example between solid fuel usage and perceived health impact and gender-related disconnect need to be understood.95 Co-developing local solutions with the community was important for success, including using sensory perceptions of mostly invisible air pollution to improve understanding. The Cleaner Cookstove Implementation Tool and the Clean Fuel Implementation Tool were proposed.94

FRESH AIR also revealed the socioeconomic impact of CRD in low-income settings: in particular, the “presenteeism” phenomenon where people attend school and work but their productivity drops; the desire among communities to improve the quality of the air they breathe. Where system-level changes are needed, making the case for change in resource-limited settings can be a major barrier.