3.1. Levels of carbonyls in indoor environments
3.1.1. Faculty of chemistry
The Faculty of Chemistry consists of several buildings, and the laboratories sampled are in a different building to the rest of common areas listed in Table 1. These common areas in the case of Inorganic and Organic Chemistry Departments are halls where offices and laboratories are around or near them.
In the laboratories sampled, the formaldehyde levels varied between 4 and 23.5 μg m-3 depending on the laboratory, while those of acetaldehyde varied from 1.3 to 13.2 μg m-3. The class practical laboratory showed the highest formaldehyde and acetaldehyde concentrations, while the lowest concentrations were found in Research Laboratory 1. The difference between Research Laboratory 1 and the other two laboratories (Research Laboratory 2 and class practical lab) was the ventilation, a fact that could explain the differences in the concentrations of the two aldehydes . Although literature information is hardly available for carbonyl compounds measured in laboratories, these measurements are in the range of those determined in research laboratories and class practical laboratories in an academic institute in Fortaleza, Brazil , and are lower than those determined in Rio Grande University in Brazil  for a research laboratory and class practical laboratories as shown in Table 2 for comparison.
|Average ± SD||Min||Max||Average ± SD||Min||Max||Reference|
|Ciudad Real (urban area)|
Research Laboratory 1
|Indoor||11||7.7 ± 3.0||4.0||11.6||5.3 ± 2.1||1.3||7.6|
|Outdoor||10||1.5 ± 0.4||0.9||2.3||1.7 ± 0.6||0.8||3.1|
|Research Laboratory 2||2||18.8 ± 4.0||16.0||21.6||5.5 ± 0.8||4.9||6.0|||
|Class practical lab||2||19.6 ± 5.6||15.6||23.5||1.2 ± 1.4||11.2||13.2|||
|Office||4||11.3 ± 2.0||8.5||13.3||5.7 ± 1.3||5.0||7.7|||
|2||9.2 ± 0.6||8.8||9.7||2.0 ± 0.2||1.9||2.1|
|Organic chemistry (2nd floor)||2||17.8 ± 0.6||17.4||18.2||2.9 ± 0.2||2.7||3.0|
|Library (2nd floor)||2||12.7 ± 0.9||12.1||13.3||2.4 ± 0.07||2.3||2.4|
|Reception (1st floor)||2||11.5 ± 0.3||11.2||11.7||2.3 ± 0.006||2.3||2.3|
|Hall (1st floor)||2||6.0 ± 0.3||5.7||6.2||1.5 ± 0.03||1.5||1.5|
|Coffee area (1st floor)||2||6.1 ± 0.3||5.9||6.2||1.6 ± 0.2||1.4||1.7|
|Outdoor||2||1.7 ± 0.4||1.4||2.0||1.8 ± 1.7||0.6||3.0|
|University library||This study|
|Ground floor||8||11 ± 1.9||8.4||14.1||5.2 ± 0.9||4.2||6.8|
|First floor||8||10.6 ± 1.6||8.6||14.2||5.05 ± 0.8||4.0||5.9|
|Second floor||8||11.1 ± 1.4||8.4||15.0||5.1 ± 0.7||4.2||6.1|
|Outdoor||4||2.1 ± 0.4||1.6||2.6||2.2 ± 0.5||1.6||2.9|
|Kitchen||2||43.7 ± 2.2||42.1||45.2||21.5 ± 4.4||18.4||24.6|
|Outdoor||6||2.2 ± 0.5||1.3||2.7||2.4 ± 1.1||0.82||3.55|
|Living room||4||46.0 ± 5.6||41.6||53.3||57.4 ± 8.5||43.3||62.2|
|Kitchen||2||38.2 ± 3.3||35.8||40.5||69.2 ± 8.2||63.4||75.0|
|Outdoor||6||3.1 ± 0.8||1.9||4.2||3.8 ± 2.0||1.2||6.5|
|Indoor||4||18.9 ± 4.7||14.7||23.1||5.2 ± 1.8||3.6||6.7|
|Outdoor||2||1.8 ± 0.8||1.2||2.3||2.1 ± 2.1||0.6||3.6|
|Puertollano (industrial area)|
|Indoor||4||35.4 ± 14.6||21.5||49.2||8.5 ± 5.4||3.7||13|
|Outdoor||2||2.0 ± 1.6||0.9||3.1||0.4 ± 0.1||0.3||0.5|
|Home 1||This study|
|Indoor||8||24.1 ± 7.5||17.4||37.1||15.2 ± 5.0||9.3||21.4|
|Outdoor||8||3.1 ± 0.6||2.4||3.8||2.4 ± 0.9||1.5||3.9|
|Home 2||This study|
|Indoor||8||74.8 ± 27.3||37.9||107.5||23.6 ± 10.7||9.6||36.3|
|Outdoor||8||3.2 ± 0.7||2.2||4.3||2.4 ± 0.9||1.2||3.5|
|Cabañeros (ecological area)|
|Outdoor||46||0.96 ±0.54||b.d.l||2.56||0.79 ± 0.49||0.13||1.89|||
Formaldehyde and acetaldehyde concentrations (µg m-3) in indoor and outdoor environments
In the office, the levels of formaldehyde and acetaldehyde were 11.3 and 5.7 μg m-3, respectively. Formaldehyde and acetaldehyde concentrations were quite similar to those obtained for the offices with no smokers of a public building in Rome  and were lower than the levels found in the offices of Rio Grande University . The values for formaldehyde are also in the range of those measured in offices from different countries from Europe .
|Rio Grande University, Brazil||32.3-41.0||18.3-26.2||Office|||
|Fortaleza, Brazil||0.32-81.6||1.2-4.43||Research laboratories|||
|Rio Grande University, Brazil||96.5||79.4||Research laboratory|||
|Fortaleza, Brazil||3.78-60.75||1.4-3.18||Class practical laboratories|||
|Rio Grande University, Brazil||56.5-161.5||38.1-91.2||Class practical laboratories|||
|Helsinki, Finland||8.1-77.8||3.7-41.5||Living room|||
|Strasbourg, France||6-93||0-66||Living room|||
|10 European cities||14.4 ± 4.9-30.7 ± 17.8||9.6 ± 2.3-15.8 ± 5.4||Homes|||
|11 European cities||5.6-49.7||1.4-29.1||Schools and kindergarten|||
Formaldehyde and acetaldehyde concentrations measured in different indoor environments in previous studies
Regarding common areas, formaldehyde and acetaldehyde concentrations ranged from 5.7 to 18.2 μg m-3 and from 1.4 to 3.0 μg m-3, respectively. The lowest levels for both aldehydes were found on the first floor (hall and coffee area). The coffee area is located on the right side of the hall, and it is not separated from this by any wall. The hall has an extension of approximately 350 m2, and there is not any furniture; there are some tables and chairs and two vending machines in the coffee area only. Because of this, it seems reasonable that the low levels of both aldehydes are found here. The highest levels were registered on the second floor especially in the Organic Chemistry Department, where the levels were similar to those obtained in the research laboratory 2 and the class practical laboratory placed in another building. This fact could be due to the reactions between the variety of organic solvents employed and ozone, but ozone levels are low usually in indoor environments, and they have not been measured in this study. Therefore, care must be taken when assuring this. The library is located beside the Organic Chemistry Department, which could influence the levels of formaldehyde found here. The reception is located on the first floor and presents levels of formaldehyde and acetaldehyde higher than those found on the other two sampling points of the first floor, maybe due to the presence of a big copying machine in the room and the emission of ozone with the consequent reaction with the unsaturated VOCs in the air (such as cleaning products) .
Not only formaldehyde and acetaldehyde have been measured in these common areas of the Faculty of Chemistry but also other carbonyls have been identified and quantified and are shown in Table 3. These are acrolein-acetone, crotonaldehyde, benzaldehyde, pentanal, p-tolualdehyde, and hexanal. The most abundant carbonyl was acrolein-acetone (both appear together in the chromatogram) with a range of concentration between 14.2 and 58.3 μg m-3. The rest of carbonyls present concentrations between 0.2 μg m-3 for benzaldehyde and 4.1 μg m-3 for hexanal (without taking into account formaldehyde and acetaldehyde explained above).
|Carbonyl||Inorganic hall||Organic hall||Library||Reception||Hall first floor||Coffee area||Outdoor|
Carbonyls measured and their concentrations (in µg m-3) in the common areas of Faculty of Chemistry
The indoor/outdoor ratio is generally used to infer penetration to indoor environments and indoor sources. An I/O ratio lower and close to one indicates more outdoor sources . In all sites, air concentrations for formaldehyde inside the buildings were between 5 and 13 times higher than outside, which appears to indicate that strong indoor sources exit, clearly determining indoor air concentrations. The same conclusion can be obtained for the pair acrolein-acetone or p-tolualdehyde (I/O = 8-20) and hexanal (I/O = 13-34), but the indoor levels of these two last carbonyls are low in all sampling sites. For the rest of compounds, the main source could be from outdoor even acetaldehyde with a ratio of 2.4-4.4.
In the bar of the campus (about 200 m2), the concentration of acetaldehyde was much higher than that of formaldehyde. The higher acetaldehyde value could be due to the cigarette smoke and combustion processes such as those carried out in the kitchen of the bar, although acetaldehyde is also present in many foods and alcoholic drinks [3, 5]. These levels of carbonyls are lower than expected. Hodgson et al.  estimated that cigarette smoking contributed from 57% to 84% to the total formaldehyde levels in the smoking areas of cafeterias where the formaldehyde concentrations varied between 5 and 42 μg m-3 when 20-100 cigarettes were being smoked. The combustion of 5-10 cigarettes in a room of 30 m2 increases the formaldehyde concentrations from 240 to 600 μg m-3 . However, formaldehyde levels measured in the bar were low and similar to those found in the teacher’s office and in Research Laboratory 1 probably due to mechanical ventilation and the great surface of more than 200 m2 of the bar.
3.1.3. University campus library
Carbonyl concentrations were also measured in the University Campus Library since paper can emit large amounts of formaldehyde . Six reading rooms of the library and a sampling point outdoor were monitored during four consecutive weeks from February 6 to March 5, 2011. Two passive samplers were placed in two reading rooms on the ground floor and the same on the first and the second floor. The total samples were 28 between indoor and outdoor sampling.
The carbonyl compounds identified in the library were formaldehyde, acetaldehyde, acetone-acrolein, propanal, crotonaldehyde, butanal, benzaldehyde, pentanal, p-tolualdehyde, and hexanal, and their average concentrations were as follows (in µg m-3): 10.9 ± 1.3, 5.1 ± 0.2, 12.4 ± 2.4, 2.5 ± 0.1, 0.4 ± 0.1, 15.4 ± 1.3, 0.55 ± 0.05, 2.9 ± 0.4, 0.8 ± 0.2, and 6.6 ± 1.1, respectively (quoted errors correspond to the standard deviation). Butanal appeared overlapped with an interference (no carbonyl compound) in the samples. Their high levels obtained with respect to the rest of carbonyls together with the UV spectrum indicate that butanal appears overlapped with other carbonyl compound may be 2-butanone. Both compounds appear together in the chromatogram when a standard is introduced under the analytical conditions. Therefore, butanal has not been considered for the discussion. The most abundant carbonyls were acetone-acrolein and formaldehyde. Their concentrations in the individual samples varied from 8.6 to 17.3 µg m-3 and from 8.4 to 15 µg m-3, respectively.
|Carbonyl||Ground floor||First floor||Second floor||Outdoor|
|Formaldehyde||11.0 ± 1.9||10.6 ± 1.9||11.1 ± 2.2||2.1 ± 0.4|
|Acetaldehyde||5.2 ± 0.9||5.1 ± 0.8||5.1 ± 0.7||2.0 ± 0.5|
|Acrolein-acetone||15.0 ± 1.9||11.1 ± 1.7||11.2 ±1.8||3.8 ± 0.8|
|Propanal||2.4 ± 0.3||2.6 ± 0.5||2.6 ± 0.4||1.0 ± 0.1|
|Crotonaldehyde||0.4 ± 0.1||0.5 ± 0.3||0.4 ± 0.3||0.7 ± 0.1|
|Benzaldehyde||0.5 ± 0.1||0.6 ± 0.1||0.6 ± 0.1||0.3 ± 0.06|
|Pentanal||3.3 ± 1.0||2.6 ± 0.4||2.7 ± 0.6||1.0 ± 0.1|
|p-tolualdehyde||0.9 ± 0.5||0.7 ± 0.1||0.8 ± 0.3||0.7 ± 0.3|
|hexanal||7.7 ± 2.0||5.9 ± 0.9||6.1 ± 0.9||1.7 ± 0.07|
Levels of carbonyls (in µg m-3) measured in the Campus University Library together with standard deviations
Generally, the concentrations of the carbonyl compounds found in the different reading rooms of the library were low. These values are in the range of those obtained in 20 university libraries in Strasbourg (France) , where the levels found ranged from 8.6 to 94.5 for formaldehyde, from 3.7 to 25.9 µg m-3 for acetaldehyde, from 2.1 to 58.8 µg m-3 for hexanal, from 0.2 to 5.3 µg m-3 for benzaldehyde, and from 0.7 to 16.3 µg m-3 for propanal. Our measurement values for formaldehyde are also in the range of those reported by Fantuzzi et al.  for 16 libraries of the university of Modena (Italy), which ranged from 1.7 to 67.8 µg m-3 with an average value of 32.7 ± 23.9 µg m-3 (see Table 2). However, the average concentrations for formaldehyde calculated in this study are lower than the studies mentioned above. Formaldehyde, acetaldehyde, propanal, benzaldehyde, and hexanal have usually been detected in libraries .
Table 4 shows the concentrations of all carbonyls measured on the different floors of the library. There are not differences between the concentrations measured of the carbonyl compounds in the different reading rooms of the different floors of the library. The indoor/outdoor ratios vary from 0.65 crotonaldehyde, which indicates mainly outdoor sources to 5.1 for formaldehyde indicating mainly indoor sources. Other I/O ratios that could suggest indoor sources but are not strong are 3.3 for acetone and 3.8 for hexanal.
3.1.4. Private houses
Formaldehyde and acetaldehyde measurements were performed simultaneously in the living room and outdoor in Ciudad Real and Puertollano; aldehydes in the kitchens were also measured only in Ciudad Real (see Table 1). Homes were ventilated as usual by opening the windows during the sampling. Two samples were collected at the same time in Ciudad Real, while the samples were collected during 8 consecutive weeks in Puertollano. All houses were located in an area of moderated traffic. Approximate home ages are between 1 and 8 years old in Ciudad Real and Puertollano; only home 2 in Ciudad Real is more than 10 years. This information is important since building materials can emit formaldehyde, but these emissions generally decrease with the age of the house. Smoking people are present in one house of each town.
Formaldehyde indoor air concentration was in average 24.5 and 15.3 times higher than the concentration outdoor in Ciudad Real and Puertollano, respectively, while acetaldehyde was 11.5 and 7.9 times higher in Ciudad Real and Puertollano, respectively. These ratios are similar to those found by Geiss et al.  for a new manufacturer house (20.2 for formaldehyde and 9.2 for acetaldehyde); however, the homes in Ciudad Real are more than five years old and between 1 and 8 in Puertollano. This fact indicates that strong indoor sources existed, clearly determining indoor air concentrations and denoted that indoor sources were dominant for these compounds. Indoor materials such as consumer products, furniture, and decorations are important indoor sources [38-41]. Carbonyls such as acetaldehyde are generated from smoking . A smoker person lives in home 2 (Ciudad Real) and presented the highest acetaldehyde levels in both kitchen and living room. Home 1 (Puertollano) is also the home of one smoker, but the frequency of smoking is low; in this case, the levels of acetaldehyde are even lower than the levels obtained in the homes of nonsmokers. Except for home 2 from Ciudad Real, formaldehyde levels were higher than the acetaldehyde ones. Bruinen de Bruin et al.  measured formaldehyde and acetaldehyde concentrations in residential environments in ten European cities, demonstrating mean levels in the range of 14.4 ± 4.9 μg m-3 (Dublin) to 30.7 ± 17.8 μg m-3 (Arnhem) for formaldehyde and from 9.6 ± 2.3 to 15.8 ± 5.4 μg m-3 (Budapest) for acetaldehyde, thus confirming low concentrations of aldehydes in European homes. Another recent study measured formaldehyde and acetaldehyde inside the homes of Strasbourg (France) . The indoor levels were in the range of 6 to 93 μg m-3 for formaldehyde and from 0 to 66 μg m-3 for acetaldehyde. Therefore, the mean concentration of formaldehyde in the indoor air in the present work is in the range of those found in other European cities such as Strasbourg or Helsinki (8.1-77.8 μg m-3)  (see Table 2). Indoor acetaldehyde had also a mean concentration similar to that obtained in other European cities (see Table 2), except for home 2 in Ciudad Real wherein concentration is the highest with an average of 57.4 μg m-3. This concentration is in agreement with some living rooms in Strasbourg , where the range of concentrations was from 0 to 66 μg m-3 for acetaldehyde.
In the case of concentrations of aldehydes measured in the kitchens of Ciudad Real, formaldehyde presents practically the same level for both homes. Our mean value is approximately 40% higher than that reported in previous studies  and is in the range of that reported in reference  for a study performed in Bari (Italy). For acetaldehyde, the mean concentration registered in the kitchen home 2 is higher than those reported in other European cities.
According to the World Health Organization, the lowest formaldehyde concentration that has been associated with nose and throat irritation after short-term exposure is 100 μg m-3 . Therefore, it represents the recommended maximal value. In our study, there is no such place with formaldehyde values exceeding this level. Despite that, it is important to note that the sampling period was 14 days for the Analyst® passive sampler (homes in Ciudad Real) and 7 days for Radiello® (homes in Puertollano). Thus, our results express an average concentration over a long period and do not provide information about exposure peaks, which must be higher than the average concentration measured. However, other recommended values exist, which were exceeded in our study. For example, the indoor formaldehyde concentrations in the homes monitored exceeded the guideline value of 30 μg m-3 proposed for the prevention of irritant effects  referred to a period of 30 min.
Hence, it is necessary to carry out an exhaustive research with more homes monitored in order to assess the health risk. In addition, taking into account the low outdoor pollution levels observed, an adequate airing of the rooms could reduce the indoor pollution by formaldehyde and acetaldehyde. Nevertheless, this study only presents preliminary results of the private homes in Ciudad Real and Puertollano due to the low number of samples. Measurements in Ciudad Real were performed only to check the viability of the Analyst® passive sampler in the indoor environment . Regarding homes in Puertollano, this study presents the preliminary results of a further research. There are not clear differences between the levels of formaldehyde and acetaldehyde registered in the urban and industrial area.
Two schools were sampling for the present study, one in Ciudad Real and one in Puertollano located in the same area as the homes sampled. Samples were simultaneously collected in two classrooms from each school and outside the building. Every classroom has an area of about 50 m2, and there are between 20 and 27 children. The school in Puertollano is more than 20 years old, and the classrooms have not been renovated recently, while in Ciudad Real, the school is also old but the classrooms were renovated 7 years ago. The ventilation in both schools is natural by opening the windows during 15 min per day.
The concentration of formaldehyde and acetaldehyde was slightly higher for the industrial area 35.4 ± 14.6 and 8.5 ± 5.4 μg m-3, respectively, versus 24.3 ± 4.7 and 6.2 ± 1.8 μg m-3, respectively, for the urban area. The concentrations of formaldehyde were more abundant than the acetaldehyde ones. In both schools, the air concentrations for both aldehydes inside the buildings were higher than outside. The indoor/outdoor ratios for formaldehyde and acetaldehyde in Ciudad Real were 13.5 and 2.9, respectively, and were higher for both aldehyde in the school from the industrial area 17.7 and 21 for formaldehyde and acetaldehyde, respectively. Except acetaldehyde in the school from Ciudad Real, formaldehyde and acetaldehyde are mainly indoor pollutants derived from indoor sources.
Our data for formaldehyde are similar or a bit higher than the levels registered in fourteen schools in Lisbon (Portugal) . The levels of both aldehydes are in agreement with the concentrations registered in eleven European cities  (see Table 2).
The levels of formaldehyde exceeded the guideline value of 30 μg m-3 proposed for the prevention of irritant effects  in the industrial area. High levels of formaldehyde are likely associated with the age of building and renovating activities of old buildings. However, both schools are more than 10 years old, and they did not have recent renovation therefore, these levels are due to other sources.
3.2. Levels of carbonyls in outdoor environments
In all sampling sites, formaldehyde and acetaldehyde were present in both indoor and outdoor air. Generally, in all sites, air concentrations for these aldehydes inside the buildings were higher than outside. Mean values for formaldehyde in Ciudad Real varied between 1.5 and 3.1 µg m-3, while acetaldehyde varied between 1.7 and 3.8 µg m-3 for the sampling period of December-February. Outdoor acetaldehyde concentrations were similar to those of formaldehyde. In the case of the industrial area, formaldehyde varied in the range of 2 (in March) and 3.2 µg m-3 (May-June) and acetaldehyde varied between 0.4 (in March) and 2.4 µg m-3 (May-June), being formaldehyde levels slightly higher than the acetaldehyde ones. Similar values were found in samplings conducted in rural and semirural areas [50-52].
Formaldehyde/acetaldehyde ratio has been proposed as an indicator of the biogenic source of formaldehyde  and can vary between 1 (urban area) and 10 (deciduous forest). This ratio for Ciudad Real and Puertollano is between 0.9 and 1.3 reflecting typical values for urban air. The formaldehyde/acetaldehyde ratio for the sample taken in the school in Puertollano gave a value of 5, which could imply a biogenic contribution in the early spring.
A study about levels of carbonyls was carried out in the ecological area of Cabañeros National Park . Twelve compounds were identified and quantified: formaldehyde, acetaldehyde, acetone-acrolein, propanal, crotonaldehyde, butanal, benzaldehyde, isopentanal, pentanal, o-tolualdehyde, m/p-tolualdehyde, and hexanal (the sum of m/p-tolualdehyde and acetone-acrolein was reported because they could not be well separated by the analytical method). The most abundant carbonyls were hexanal, acetone-acrolein, formaldehyde, and acetaldehyde.
Because of the study covered all seasons, August 2010 was the most carbonyl-polluted month followed by July 2011 and September 2010. October and November 2010 were the months with lower concentration of carbonyl compounds showing an apparent seasonal variation with maximum values observed in summer months. The concentrations of the carbonyls were as follows: acetone-acrolein mixing ratios ranged from 0.35 in February to 4.52 µg m-3 in June 2011, with an average of 1.78 µg m-3, while ambient levels of hexanal varied from 0.67 µg m-3 in October to 1.72 µg m-3 in April; the average concentration during the sampling period was 1.06 µg m-3. Formaldehyde was in the range of values below detection limit and 2.56 µg m-3 in October and June, respectively, with an average concentration of 0.96 µg m-3 and acetaldehyde varied from levels below detection limits in February, March, and April to 1.89 µg m-3 in June. The concentrations of other carbonyls ranged from non-detected to 1.18 µg m-3.
The seasonal cycles obtained for formaldehyde, acetaldehyde, and acetone-acrolein with respect to the other carbonyls suggest different formation mechanism and sinks compared to the others. This could be due to photochemical processes (oxidation of biogenic and even anthropogenic hydrocarbons that arrive to the area under determined meteorological conditions) and also the direct emission from vegetation. For example, two Mediterranean tree species such as Quercus ilex (holm oak), one of the most abundant oak in the study area, and Pinus pea (Italian stone pine) emit formaldehyde and acetaldehyde [54, 55].
The formaldehyde and acetaldehyde concentrations measured at Cabañeros National Park are in the same range or lower than the levels reported in other forests or rural areas (Germany  and Brazil ). Our data are similar to those reported in the small village of Covelo in Portugal, considered as a rural/forest site .
On the other hand, the levels found in the ecological area during the months of February and March for formaldehyde (0.51 and 0.59 µg m-3) and acetaldehyde (0.33 and 0.29 µg m-3) are lower than those registered in the urban (outdoor samples collected at the same time as common areas of Faculty of Chemistry or University Library) and industrial area (outdoor samples collected in the school) during the same period. Also, the levels of both aldehydes during the months of May and June were lower (1.15-1.37 µg m-3 for formaldehyde and 0.70-1.09 µg m-3 for acetaldehyde) than the levels registered in the industrial area during the same months in the outdoor samples collected in the homes.
3.3. Health risk evaluation
Formaldehyde is classified in Group 1 by IARC. It was based on inhalation causing squamous cell carcinoma in rats and nasopharyngeal cancer in humans . Recently, the classification has been expanded with formaldehyde causing leukemia and limited evidence of sinonasal cancer in humans . The lowest concentration reported to cause sensory irritation of the eyes in humans is 0.38 mg m-3 for 4 h. Increases in eye blink frequency and conjunctival redness appear at 0.6 mg m-3, which is considered equal to the no-observed-adverse-effect level (NOAEL). There is no indication of accumulation of effects over time with prolonged exposure, and there is no evidence indicating an increased sensitivity to sensory irritation to formaldehyde among people often regarded as susceptible (asthmatics, children, and older people) .
The formaldehyde exposure-response relationship is highly nonlinear and biphasic, supporting a NOAEL that allows setting a guideline value . This means that it cannot be assumed a low-dose linear relationship for the carcinogenic effects, and therefore, the calculation of the incremental lifetime risk of cancer for formaldehyde, assuming a low-dose linear relationship, would give a value highly overestimated and it would be meaningless.
As commented above, WHO established a guideline value for formaldehyde of 100 µg m-3 that should not be exceeded for any 30-min period of the day . This short-term guideline will also prevent effects on lung function as well as long-term health effects, including nasopharyngeal cancer and myeloid leukemia .
We can compare our data with the short-term guideline value of 100 µg m-3. In our study, the maximum values registered in the different environments were below 100 µg m-3, except for home 2 in the industrial area that registered a maximum value of 108 µg m-3 during one of the 8 weeks sampled. The average concentrations for formaldehyde found in this study ranged from 6 to 75 µg m-3 in indoor environments, while outdoor concentrations were considerably lower. These values are below the guideline value considered preventive of carcinogenic effects; however, our results represent an average value during 1 or 2 weeks of exposition, and this guideline value is referred to a period of 30 min. Nevertheless, as long as formaldehyde concentrations are below 100 µg m-3, there should be no chance of developing cancer.
On the other hand, the most important way to control the indoor formaldehyde concentration is the air exchange rate and the use of low-emitting materials and products. Environmental tobacco smoke and ozone-initiated reactions of alkene compounds may contribute to temporary peak levels