7b

Environ Geochem Health (2007) 29:143–153 DOI 10.1007/s10653-006-9069-6

ORIGINAL PAPER

Environmental radon studies in Mexico N. Segovia Æ M. I. Gaso Æ M. A. Armienta

Published online: 8 February 2007
Springer Science+Business Media B.V. 2007

Abstract Radon has been determined in soil, groundwater, and air in Mexico, both indoors and outdoors, as part of geophysical studies and to estimate effective doses as a result of radon exposure. Detection of radon has mainly been performed with solid-state nuclear track detectors (SSNTD) and, occasionally, with active detection devices based on silicon detectors or ionization chambers. The liquid scintillation technique, also, has been used for determination of radon in groundwater. The adjusted geometric mean indoor radon concentration (74 Bq m–3) in urban developments, for example Mexico City, is higher than the worldwide median concentration of radon in dwellings. In some regions, particularly hilly regions of Mexico where air pollution is high, radon concentrations are higher than action levels and the effective dose for the general population has increased. Higher soil radon levels have been found in the uranium mining areas in the northern part of the country. Groundwater radon levels are, in general, low. Soil-air radon contributing to indoor atmospheres and air pollution is the main source of increased exposure of the population. N. Segovia
M. A. Armienta (&) Instituto de Geofisica, UNAM, Ciudad Universitaria, Circuito Exterior, C.U., Mexico, DF 04510, Mexico e-mail: [email protected] M. I. Gaso ININ, Ap. Post 18-1027, 11801 Mexico, DF, Mexico

Keywords Soil
Groundwater and air radon
Enhanced exposure

Introduction The potential hazards posed by exposure to alpha radiation from air radon have been of great concern worldwide, especially associated with increased lung cancer risk. Causal association of exposure to 222Rn and lung cancer has been demonstrated in epidemiological studies performed on cohorts of miners (Doi et al., 2001). Domestic radon has been identified as the most important environmental risk factor for lung cancer. Wichmann et al. (2002) report that 7% of all lung cancers in Germany could be because of indoor radon. Understanding of environmental, physical, physiological, and biological conditions is crucial to devising methods for estimation of doses on the basis of exposure to radon and radon decay products. The radioactive dose from radon is the main natural dose received by mankind. Dosimetry of radon is a very complex problem that has been studied intensively during the last decade (UNSCEAR, 2000). Radon monitoring and indoor radon concentration levels have been of scientific and technological interest because of their several applications as useful tools in studies of hydrology, geology, oceanography, earthquake

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prediction, and earth sciences. Radon is produced within and on the surface of rock grains that contain uranium and its daughters in secular equilibrium. Radon atoms which escape from these grains are mostly produced by alpha recoil from radium disintegration of atoms located a few nanometers from the grain surface. When radon is released from the grains of a porous material, for example soil, it moves under diffusion and transport mechanisms. Migration is usually enhanced by transport by groundwater and terrestrial gases. Radon gas is regarded as a natural radioactive element with mobility in the crust (Corbett & Burnett, 1997). The presence of radon in the atmosphere is a consequence of its upward transfer from near surface soils and rocks. Radon concentrations in domestic and public buildings depend on soil permeability, meteorological conditions, building materials, ventilation, and occupant use of the building (Jo¨nsson et al., 1997). Radon monitoring in soil, air, and water, and around active volcanoes and seismic zones, has been performed in Mexico as an additional tool for geophysical risk assessment. Radon-in-soil surveys in Mexico were initiated in 1974 in the northern uranium mining region for uranium prospecting (Aguilar, 1981). Being a country with several active volcanoes and located in a strong seismic subduction zone, radon-in-soil-gas mapping has been systematically performed since 1981, to correlate temporal fluctuations of soil radon emanation with local telluric behavior (De la Cruz-Reyna, Mena, Segovia, Chalet, & Monnin, 1985; Segovia et al., 1989, Segovia, Mena, & Tamez, 1993, Segovia et al., 1999b, Segovia, Valdes, Pen˜a, Mena, & Tamez, 2001). The radon records also include data from Mexico City (Segovia, Pen˜a, & Tamez, 1991), the geothermal field of Los Azufres located in the North–East of the State of Michoacan (Segovia et al., 1989) and the mining zone of Zacatecas (Segovia, Pen˜a, Mireles, Da´vila, & Quirino, 1993). The first observations of the activity of radon and short-lived radon daughters in groundwater from alluvial and volcanic aquifers in Mexico were obtained by Olguin et al. (1990) and Segovia and Bulbulian (1992). Concentrations of 222Rn, natural radionuclides, and physicochemical con-

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ditions were determined in groundwater samples from wells and boreholes of the drinking water supply system of Toluca City (Olguin, Segovia, Tamez, Alcantara, & Bulbulian, 1993; Segovia et al., 1999a). In other places, for example Mexico and Michoacan states, chemical analysis of major and trace elements, and determination of radon were also performed in springs and wells from urban and agricultural zones in the Lerma river basin (Lopez et al., 2002; Alfaro et al., 2002). Measurements of radon and the chemical composition of water samples obtained from springs located around Popocatepetl, an active volcano, have been obtained over several years (Segovia et al., 2002b, 2003, 2005). Atmospheric outdoor and indoor radon surveys have been conducted at different locations in Mexico associated with a variety of research projects (Gaso, Cervantes, Segovia, & Espı´ndola, 1994; Segovia, Ponciano, Ruiz, & Godinez, 2002a, c; Chavez, Balcazar,& Camacho, 2003; FrancoMarina et al., 2001; Franco-Marina, VillalbaCaloca, Segovia, & Tavera, 2003). It has been shown that although indoor radon concentrations are mainly affected by the strength of the sources and air exchange with outdoor air, the geographical (altitude and latitude) and geological (soil of volcanic and sedimentary origin) characteristics of Mexico suggest the presence of radon-prone areas. Because of a shortage of data about indoor exposure to 222Rn at high altitudes in semi-cold sub-humid climates, the main purpose of this work was to analyze reported radon concentrations in Mexico, to gain an overview of the indoor radon problem in the country. All populations are at risk of exposure to chemicals. Toxic chemicals pervade our society and there is almost no way of avoiding at least some exposure to toxic compounds from household products, for example cleaners, disinfectants, paints, pesticides, food treated with pesticides, natural materials such as radon gas, and water, air, and soil contaminated by release from industry and other facilities into the environment. Several elements, for example arsenic, chromium, copper, nickel, and zinc are frequently involved in environmental toxicity problems. Although it is not possible to quantify the hazards and deleterious effects associated with the trace elements in

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common use, some elements are clearly a more serious problem than others. It is necessary to determine how much ‘‘contamination’’ merely reflects the preexisting natural background level. This question of natural background levels has important economic implications. Two or more toxic chemicals can exert a combined effect (synergistic effect) which is much greater than the sum of effects of the chemicals (Selinus, 2004). In Mexico there is little information about the public health risk of radon. Air pollution in Mexico City is a huge environmental problem and it is possible that cumulative exposure to radon indoors, with other, synergistic effects, for example indoor aerosols, tobacco smoke, and exposure to industrial and urban pollution, could severely affect the population’s health. High indoor radon, and smoking tobacco, both cause cancer. Smoking has become an important public health problem in Mexico, with more than 14 million smokers. Malignant tumors, many associated with direct exposure to tobacco smoke, are the second largest cause of mortality in the general population, after cardiovascular diseases; lung cancer is the leading neoplasm. Chihuahua has a lung cancer mortality rate double that of the national average, as have other northern Mexican states (INEGI, 2001).

The Study Sites Mexico is almost 2,000,000 km2 in extent. Fourteen active volcanoes are located mainly along the Mexican Neovolcanic Belt. The southern part of the Pacific coast is one of the most seismologically active areas in the world, because of the subduction of the Cocos and Rivera Plates under the North America Plate. The recurrence period for large earthquakes in this area is estimated to be in the range of 30–70 years. A seismic gap has been identified at the State of Guerrero where a large earthquake of magnitude Ms 8.1 occurred in 1985 and another, up to Ms 8.2, was predicted (Singh, & Mortera, 1991). More than a half of the population of Mexico is settled in the volcanic and seismic zones of the country at altitudes between sea level and 2600 m.

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Indoor radon concentration data are available mostly from Central Mexico where more than 30 million inhabitants are settled. Mexico City is located in an originally closed hydrological basin that was artificially opened at the beginning of the seventeenth century. The basin extends over an area of 7,500 km2 and has approximately 20 million inhabitants and three million cars. It has three times the population density of Paris, and four times that of London. Only Bombay, Calcutta, and Hong Kong have higher densities. The lowest part of Mexico City, the lake zone, has an average altitude of 2240 m, and is surrounded by mountains, the highest being Popocatepetl and Iztaccihuatl (active and dormant volcanoes respectively) at the eastern part of the city reaching altitudes of 5465 and 5230 m, respectively. Dormant volcanoes are also found in the southern part of the city. Popocatepetl volcano is located between two of the most populated valleys of the country, the Mexico City and the Puebla City valleys. The two cities are located 60 and 50 km, respectively, from the volcano. Toluca City, the capital city of the State of Mexico is the highest altitude capital city of the country (2600 m); it has three million inhabitants and is located approximately 70 km west of Mexico City. A radon survey was also carried out in the city of Guadalajara (the third biggest city in Mexico), located in a valley of the western part of the Mexican Neovolcanic Belt. The geographical latitude and longitude of the main regions studied are shown in Fig 1.

Materials and Methods Radon measurements in Mexico wereperformed using long-term exposure passive detectors and short-term evaluations with real-time active detectors. Environmental radon measurements in soil and air were performed mainly with passive detectors, for example solid-state nuclear track detectors (mainly LR 115 and CR 39). Track detector methods are based on tracks formed in a plastic because of the energy released by the alpha particle in the material. After exposure, the detectors are chemically etched and the number of alpha tracks per unit surface is counted by

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Fig. 1 Locations of the regions studied. Gray points indicate cities included in Table 1, and symbols indicate volcanoes

means of microscopic measurements (direct observation, spark counting, and image detection systems). Track detectors have been used for soil radon measurements at a depth of 70 cm (Segovia, Seidel, & Monnin, 1987, Segovia et al., 1989, 1991). Aware Electronics, RM-60 and RM-70 radon and decay products detection systems (Quirino et al., 2002), with alpha scintillation probes (Armienta, Varley, & Ramos, 2002) have also been used to measure in situ radon in soil air. The radon content of groundwater was measured by using the liquid scintillation method for water samples and detection equipment such as the Packard No. 4530 and the Tri-Carb 2700TR (Segovia, & Bulbulian, 1992; Lopez et al., 2002). Track detectors were also used for determination of radon in ground water wells (Espinosa, Golzarri, & Cortes, 1991). Short-term radon determinations in groundwater have been performed with an automatic Clipperton type probe based on a silicon diode detector (Morin, Seidel, & Monnin, 1993). Track detectors using different container designs and automatic silicon-based radon monitors (Gaso et al., 1994), have been used in atmospheric surveys (Segovia et al., 1991, Franco Marina et al., 2003; Espinosa, & Gammage, 2003). Honeywell

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A9000A and Alphaguard radon monitors have also been used to measure exposure times between 1 and 24 h for several days at specific sites. The Honeywell radon monitor is a continuous radon monitor that samples radon gas concentrations using a passive radon chamber design. The detectors were calibrated in radon chambers certified by the EPA (USA). For representative radon concentrations, equilibrium, and occupancy factors, and dose coefficients in terms of equilibrium equivalent radon concentration (EEC), the annual effective doses were derived (UNSCEAR, 2000) by use of the equation: HE = CRn Eq T CFRn 106

ð1Þ

where HE is the annual effective dose by inhalation of radon (mSv y–1); CRn is the radon concentration in air (Bq m–3); Eq is the equilibrium factor (0.4 and 0.6 for indoor and outdoor environments, respectively); T is the occupancy factor (7000 h y–1 and 1760 h y–1 for the indoor and outdoor environments to the general public); CFRn is a dose conversion factor for radon (9 nSv Bq–1 h–1 m3 and 6 nSv Bq–1 h–1 m3 in

Environ Geochem Health (2007) 29:143–153

terms of the EEC and equilibrium factors of 0.4 and 0.6 for indoor and outdoor environments respectively). The EEC can be converted to the potential alpha energy concentration (PAEC) when desired, by use of the relationships (UNSCEAR, 2000): 1 Bq m–3 = 5.56 10–6 mJ m–3 = 0.27 mWL (222Rn).

Results Soil gas radon measurements Many radon-in-soil surveys have been conducted to obtain an estimate of radon concentration levels in Mexico. The values obtained, from stations at more than 150 sites, have been presented in a map covering approximately one third of the territory of Mexico (Segovia, Tamez, & Mena, 1992). Most of the monitoring stations were located in the Mexican Neovolcanic Belt. Radon-in-soil levels were relatively low at most of the sites; the mean value was approximately 10 kBq m–3. Higher values, compared with those in the rest of the country, were measured in northern areas. A radioactivityrich mineralization zone is located in the northeast, and in the north-west a value of 499 kBq m–3 is related to a uranium-rich zone in the State of Sonora. In the southern part of the country moderate radon levels of 18 kBq m–3 were associated with intrusive rocks and stations with enhanced hydrothermal activity. In Central Mexico and Mexico City, maximum values of 8 and 15 kBq m–3, respectively, were reported (Segovia et al., 1991). The lowest values were found in active volcanoes (Colima, El Chichon, and Popocatepetl volcanoes). The results obtained show that temperature, relative humidity, and rainfall patterns all affect movement of radon from soil into the atmosphere. Popocatepelt volcano started an eruptive phase in December 1994 with the emission of fine ash. Its eruptive history is characterized by recurrent eruptions every 1000–3000 years During 1995 the ash emitted was old material. In 1996, juvenile material started forming a dome in the southern part of the 800 m deep crater (Segovia et al., 1997). Soil radon increased from January to May, 1995, which correlated with the starting eruptive

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phase. Soil radon behavior in relation to the volcanic eruptive period 1997–1999 has also been reported (Segovia et al., 2001). On June 30, 1997, the largest event within those years occurred. The eruption was preceded by a series of volcanic earthquakes generating a plume 13 km high, and ash falls that coated the south west part of Mexico City with a 5 mm layer. Radon concentrations in the soil were low, however, and only a few peaks, not higher than 3 k Bq m–3, were observed. The largest volcanic activity observed at Popocatepetl from 1994 to 2005 occurred in December 2000January 2001, when a large lava dome was formed inside the crater and was destroyed by an explosive eruption (Segovia et al., 2003, 2005). These events had been preceded by a change in volcanic seismic activity since September, 2000. Soil radon signals increased with volcanic activity. Since mid 2000 the soil radon background level around the volcano has remained higher than in the preceding years. Soil radon levels have also been systematically determined during the last decade along the Pacific coast. During 1997 the seismicity was extremely intense along the coastal area of the State of Guerrero, and some radon peaks (between 20 and 30 kBq m–3) were observed at Acapulco. Some observed variations were correlated with a period of particularly intense meteorological perturbations which occurred as a consequence of El Nin˜o, culminating in the formation of a tropical hurricane that struck the Guerrero coast (Segovia et al., 1999b). Radon in groundwater and drinking water supply systems The first measurements of radon in groundwater were made in some samples from alluvial and volcanic aquifers. The water samples were taken from wells and springs in San Luis Potosi, and Michoacan, and Mexico states. Radon content varied depending on the sample source, reaching a maximum of 11.3 Bq L–1 at Toluca City (Segovia, & Bulbulian, 1992). Fifty-eight percent of the wells sampled contained less than 2.4 Bq L–1, confirming that water samples from alluvial and basaltic rocks usually contain low levels of radon (Olguin et al., 1993). Natural radionuclides and physico-

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chemical properties have also been determined for groundwater samples from boreholes used to supply drinking water to Toluca City. 222Rn content was low (the average value was 2.2 kBq m–3). Uranium and 226Ra concentrations in the water samples were also low. The local flow belongs to a shallow water system, recognized by its low radon content and dissolved ions compared with regional deeper groundwater flow with a longer residence time (Segovia et al., 1999a). Average radon concentrations in water samples from springs and wells of the Upper Lerma river (located in the State of Mexico) were relatively low, ranging from 0.9 to 4.99 Bq L–1 (Lopez et al. 2002). High radon concentrations (34.2 kBq m–3 and 27.3 kBq m–3) were, however, measured in groundwater and in the drinking water supply system, respectively, in Aldama, Chihuahua, in the northern part of the country (Colmenero et al., 2004).

Outdoor and indoor radon concentrations In Mexico, outdoor radon concentrations in different geological zones were quite comparable; average values were between 13 and 23 Bq m–3 (Segovia et al., 1994; Martinez et al., 1998). The small fluctuations observed can be attributed to air convection during the sampling time. Comparison of mean values from Mexico City in the spring and autumn reveals that radon concentrations in spring were 31% higher than in autumn (Segovia et al., 1991). Indoor radon concentrations were measured in family houses in several towns in the Mexican Neovolcanic Belt and in other family houses in towns in the north part of the country. The arithmetic mean and ranges of indoor radon concentrations (Bq m–3) reported for different cities in Mexico, with the effective dose calculated using Eq. (1), are shown in Table 1. The Mexican data median value for dwellings (72 Bq m–3) is higher by a factor of 1.7 than the worldwide radon concentration median value (46 Bq m–3) reported by UNSCEAR (2000). The average effective dose (2.1 ± 1.1 mSv y–1) was twice the world average effective dose (1.0 mSv y–1) for indoor environments (UNSCEAR 2000).

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The cities where the values are highest are Aldama and Chihuahua, located in the northern part of the country (Colmenero et al., 2004). In Mexico City the calculated average indoor radon concentration and range, taking into consideration all the values reported, were 73.8 and 4–300 Bq m–3 (Table 1). An adjusted geometric mean of 74 Bq m–3 (range 41–136) was reported by Franco et al. (2003). The highest adjusted geometric mean (136 Bq m–3) was found in the southwestern part of the city (the hill zone characterized by a surface layer of lava flows and volcanic tuff), where for 46% of households the estimated radon level was in excess of 200 Bq m–3. Systematically, the southwest part of the city also had the highest atmospheric pollution densities, because of the local wind patterns and because the mountains are a natural barrier to atmospheric circulation. In the rest of the city the geometric mean concentration ranged between 41 and 98 Bq m–3. The guideline action levels in chronic exposure situations involving radon in dwellings are 200–600 Bq m–3 (IAEA, 1996). The highest radon values usually occur in the early morning, between 1 and 3 am. In the night and early morning hours, atmospheric (temperature) inversion conditions often occur; these tend to trap the radon close to the ground. The lowest values were observed between 1 and 5 pm, probably because of important indoor–outdoor air exchange and the air movement associated with daily activity inside the dwellings and, therefore, with dilution of radon in the indoor atmosphere during the afternoon (Segovia et al., 2002a). In evaluation of indoor radon risk, time spent at home, outdoors, and elsewhere must be taken into account. The average lifetime risk of lung cancer for chronic exposure of the Mexican population to the estimated geometric mean of 74.6 Bq m–3 of radon at home (Table 1), with an occupancy factor of 0.6, is approximately 5 · 10–3. This value is indeed significant when compared with other risks that affect the general population, and was estimated on the basis of an average life expectancy of 77 years (80 years for females and 74 years for males), a lifetime risk factor of 2.8 · 10–4 WLM–1 as proposed by the ICRP

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149

Table 1 Average values and ranges of indoor radon concentrations (Bq m–3) and annual effective dose (mSv y–1) in different cities in Mexico Effective dose (mSv y–1)

References

Site

North West Altitude Method Indoor latitude longitude (m) used radon (Bq m–3)

Aguascalientes, Aguascalientes

21 53¢

102 18¢

1870

a

61 (39–130)

Aldama, Chihuahua Chihuahua, Chihuahua Guadalajara, Jalisco Hermosillo, Sonora Leon, Guanajuato Mexico City Mexico City Mexico City Mexico City Monterrey, Nuevo Leon Morelia, Michoacan Pachuca, Hidalgo Puebla, Puebla Puebla, Puebla Quere´taro, Queretaro San Luı´s Potosı´, San Luis Potosi Toluca, Edo. de Mexico Zacatecas, Zacatecas

28 50¢

105 55¢

1270

b

225 (29–448)

Espinosa, Golzarri, Rickards, & Gammage, 1999. 5.7 (0.73–11.29) Colmenero et al., 2004

28 38¢

106 04¢

1440

a

135 (42–273)

3.4 (1.1–6.9)

Espinosa et al., 1999

20 39¢

103 19¢

1700

a

117 (37–190)

2.9 (0.93–4.8)

Espinosa et al., 1999

29 05¢

110 56¢

210

a

91 (27–157)

2.3 (0.68–3.9)

Espinosa et al., 1999

21 19 19 19 19 25

07¢ 27¢ 27¢ 27’ 27¢ 41¢

101 40¢ 99 08¢ 99 08¢ 99 08¢ 99 08¢ 100 17¢

2400 2240 2240 2240 2240 537

a a d a a, c a

67 34 90 97 74 97

1.7 0.9 2.3 2.4 1.9 2.4

Espinosa et al., 1999 Segovia et al., 1993 Martinez et al., 1998 Espinosa et al., 1999 Franco-Marina et al., 2003 Espinosa et al., 1999

19 41’

101 10¢

1951

a

45 (4–165)

1.1 (0.10–4.2)

Espinosa et al., 1999

20 19 19 20

07¢ 02¢ 02¢ 35¢

98 44¢ 98 11¢ 98 11¢ 100 23¢

2400 2160 2160 1820

a a a a

120 (20–187) 3 (0.50–4.7) 54 (48–60) 1.4 ((1.2–1.5) 72 (49–101) 1.8 (1.2–2.6) 61 (4–163) 1.5 (0.10–4.1)

Espinosa et al., 1999 Segovia et al., 1993 Espinosa et al., 1999 Espinosa et al., 1999

22 09¢

100 58¢

1860

a

49 (4–148)

1.2 (0.10–3.7)

Espinosa et al., 1999

19 17¢

99 37¢

2600

c

47 (11–440)

1.2 (0.28–11.1)

22 46¢

102 34¢

2420

a

46 (14–86)

1.2 (0.35–2.2)

Gaso, Segovia, Pulinets, Leyva, & Ponciano 2005. Segovia et al., 1994

(20–130) (4–296) (55–300) (45–280) (41–136) (45–280)

1.5 (0.98–3.3)

(0.50–3.3) (0.10–7.5) (1.4–7.6) (1.1–7.1) (1–3.4) (1.1–7.1)

a, nuclear tracks in solids; b, activated charcoal; c, dynamic detector; d, electret passive radon monitor

(1993), and an average equilibrium factor of 0.4 between radon decay products and radon.

Discussion Changes in soil radon concentrations in Mexico have been observed along the Pacific coast in connection with the 1985, Ms 8.1, earthquake; a correlation between the number of earthquakes and average soil radon was also observed in this region. Measurements from 1993 until 2002 of soil radon and the chemical composition of groundwater around Popocatepetl volcano showed maximum soil radon values correlated with the initial phase of the eruption (end 1994–beginning 1995)

and the end of 2000, indicating that spatial and temporal variations can provide insight into the dynamics of active volcanoes and can be used as a tool for prediction of volcanic eruptions (Armienta, & De la Cruz-Reyna, 1995; Armienta, Varley, & Ramos, 2002; Segovia et al., 2002b). Radon in tap water may lead to exposure from the ingestion of drinking water and from inhalation of radon released to air when water is used. This is not a separate contribution to the effective dose, however, because the radon source from water use would have been included in measured indoor radon concentrations. Considering a radon concentration in water of 10 kBq m–3, the estimated exposure from ingestion is 0.002 mSv y–1 (UNSCEAR, 2000).

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Indoor radon concentrations in Mexico can be regarded as medium-high compared with those in other countries and are similar to those (75 Bq m–3) obtained by Bochicchio et al. (1996) in a national survey of population exposure to radon in Italian dwellings. Indoor radon levels are mostly because of the contribution from the soil under the building, and standards of ventilation. The highest indoor radon values (Table 1) were found at Aldama, Chihuahua, and Monterrey in the north of the country where radioactive minerals are found and where uranium was mined 20 years ago. The local geology and the weather characteristics, very hot in summer and very cold in winter, are mainly responsible for the high radon levels; the dwellings have poor ventilation, are heated in winter and have air conditioning in summer. In contrast, indoor radon levels in dwellings in Mexico City and in the surrounding cities are probably high because of atmospheric pollution of the region, the specific habits of their owners, the sub-soil characteristics of the city (a lake zone, a transition zone, and a hill zone), and the construction characteristics of some houses, which have dirt floors. The spatial distribution found in Mexico City shows that the city areas with very high estimated residential radon concentrations are in the proximity of the hill zones (Franco-Marina et al., 2003). Levels of ventilation in the dwellings of Mexico City are usually quite high, because of the temperate climate. Indoor radon concentrations are, on average, nearly twice as high in the winter and spring as in the rainy season (June–October), but the opposite is true for the specific activity of the associated decay products. An increase in humidity causes an increase in the average equilibrium factor (F) between 222Rn in the atmosphere and the short life-time progeny. Considering different aerosol conditions, previous estimates in Mexico (Chavez et al., 2003) indicate that the equilibrium factor between radon and daughters can change from F = 0.2 to F = 0.77. Under these conditions the annual effective dose can increase by a factor of four for the same original radon concentration, because of different aerosol condition indoors (Chavez et al., 2003). A strong effect of the size and concentration of

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aerosol particles on typical indoor domestic exposure to radon was also reported by Huet, Tymen, & Boulard, (2001). The equilibrium factor and total solid particles follow the same pattern, F increases with increasing aerosol particle concentrations (Martinez et al., 2000). Indoor aerosol particles in temperate regions may originate from urban pollution and from very local indoor sources (dust, gas stoves, smoking, fungi, etc). In Mexico City the average total solid particles indoors (213 ± 141 lg m–3) increased with time and there seemed to be a correlation between the increase in lead and the increase in total solid particles whose distribution pattern followed wind directions (Martinez et al., 2000). When indoor PM10 particles were monitored in the bedrooms of Mexico City dwellings and in smoker’s offices spot values were as high as 420 lg m–3 and 550 lg m–3, respectively. It is worth mentioning that the outdoor PM10 intervention level established by Mexican legislation in 24 h once in a year is 150 lg m–3 (Segovia et al., 2002a). Indoor average concentrations of Cr (0.99 ± 0.18 lg m–3) and Ni (0.80 ± 0.14 lg m–3) are, however, much higher than the values of 0.01–0.03 lg m–3 and 0.02 lg m–3 respectively, for industrial cities (Martinez et al., 2000). Toxic metals such as Cr and Ni have several effects, but are primarily carcinogenic. The indoor average concentration of Pb (4.89 ± 0.95 lg m–3) is usually above the Mexican Technical Norm NOM-026-SA 1 (average 1.5 lg m–3 over 3 months). Lead is antagonistic to normal central nervous system function and is an inhibitor of macrophage alveolar activity. Such contaminants are adsorbed on PM10 particles and then deposited on the low respiratory system, primarily on alveola. Chromium, lead, and nickel are among the most hazardous air pollutants (HAPs) and are examples of carcinogenic toxicants (lung cancer) (EPA, 2006). Brauer, Avila-Casado and Fortou (2001) reported that lung tissue samples taken at autopsy from Mexico City residents contained larger quantities of fine and ultrafine particles than autopsy lung tissue samples taken from residents of Vancouver, Canada. Fortoul et al. (1996, 2001) reported higher concentrations of

Environ Geochem Health (2007) 29:143–153

metals (cadmium, copper, cobalt, nickel, and lead) in autopsy lung tissue obtained from Mexico City residents who died in the 1990s than in those who died in the 1960s, and concluded that metals in the ambient air of Mexico City had increased over time. These authors also indicated that fuel-gas deposits from oil-fired furnaces—particularly those using oil from Kuwait, Iran, Iraq, Venezuela, and the Mexican Gulf—contained high concentrations of vanadium. The main source of vanadium in city air is probably combustion of the gasoline that fuels vehicles. Symptoms of intoxication from vanadium inhalation include eye, lung, and nose irritation. This trend implies that vanadium in ambient air is increasing and may pose a human health risk. Mexico City Valley consumes large amounts of vanadium-rich fuels, and high retention of fine and ultrafine particles in the lungs of the city’s residents has been demonstrated. As far as the authors are aware, no epidemiological studies have been conducted in Mexico to evaluate the contribution to the incidence of malignant tumors of radon and other atmospheric indoor pollutants. Atmospheric pollution in Mexico City is responsible for much respiratory disease among the population, however, especially during the dry season (November– May). Because lung cancer is the result of cumulative exposure to carcinogens, for example radon and other atmospheric pollutants, it is important to obtain more information about the magnitude of this risk in the country. Acknowledgements The authors acknowledge Mr. F. Montes for technical assistance, and partial financial support from CONACyT project 40858.

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