S Ramstad, D Bratlid, T Christensen, A Johnsson

Abstract

The 50 Hz magnetic flux densities created by some typical equipments at a neonatal intensive care unit were measured. A detailed study was made of the magnetic flux densities in an incubator, two types of radiant warmers, a heated water bed and a phototherapy unit (BiliBed). The mean flux densities in an incubator was typically in the region 0.2 - 1 μT (maximum around 1.5 μT), for the radiant warmer less than 0.05 μT at the bed level and for the phototherapy unit 0.15-2.6 μT. Although the heating pad under the water mattress produced no fields, the control unit created levels up to about 2 μT at the patient position when located at the bed end. Compared to typical field levels found in residential houses the measured levels are stronger by two orders of magnitude. Moving the electrical control units away from the infants can strongly reduce the fields, as shown by the measurements.

The possible health effects of exposure to magnetic fields from the electric energy distribution systems are currently under intense study. The investigations on a possible connection between exposure to 50/60 Hz magnetic fields and an increased risk of leukaemia and brain cancer among children^1-5 emphasise the importance of proper studies of children's electromagnetic environments in this context. The magnetic flux levels frequently discussed are typically in the order of 0.1 μT.¹

Special focus has been directed to the magnetic field levels close to high voltage distribution lines⁶ but also field levels at working places and offices has attained interest.^7-10 A review of the magnetic field levels under different conditions have been published by Shaw and Croel.¹¹

One group of particular interest is the fetus and the newborn infant. There are, however, fairly few epidemiological studies focusing on developmental effects caused by the 50/60 Hz magnetic fields. Juutilainen et al.¹² showed that women with early pregnancy losses (sub clinical) were more likely to live in residences with strong 50 Hz magnetic fields than were woman with normal pregnancies. Some reports have also suggested that childhood brain cancer might be correlated with prenatal use of electric blankets or paternal employment in electrical occupations.^13,14

The environment around premature infants is of special interest in this context. Compared to infants in the home environment, premature infants in intensive care units are often exposed to special electrical medical equipment producing electromagnetic fields. The distance between the infants and the equipment can be quite short and the exposure time can be appreciable. In a study by Anger,¹⁵ the electromagnetic field environment of premature infants was discussed and measured in some of the incubators used in an intensive care unit. The levels of the magnetic flux densities were reported to be typically around 0.1 - 0.5 μT. Also other studies confirm fairly high 50/60 Hz magnetic field levels.^16-18 In a previous report¹⁸ we have presented detailed results from measurements of the magnetic flux densities in different types of incubators and overhead phototherapy units, as well as levels created by the electrical components of different monitors and ventilators. We also presented data of actual electromagnetic fields at patient head level during ongoing intensive care treatment of premature infants. Furthermore, we indicated that significant reduction in field levels can be obtained by a simple remodelling of a standard incubator.^18,19

Further investigations and precise measurements of the electromagnetic environment of premature infants seem important. Also, epidemiological follow-up studies might be of interest to see if long term effects from field exposures can be ascertained. In the present report we extend our studies and focus on incubators as well as on the electromagnetic levels from radiant warmers, heated water mattresses and a bed phototherapy unit.

Methods and Materials

Magnetic field measurements

The magnetic flux densities were measured with an EMDEX II Magnetic Field Meter (Enertech Consultants, Campbell, CA, USA). The RMS values of the flux densities (B_x, B_y and B_z) in the x, y and z directions are logged via three orthogonal coils. The magnetic fields were recorded in the frequency range of 40 - 800 Hz (broad band).

Measurements were performed over a 5x5 cm grid at the mattress surface. Samples were taken every 3 s for a total of 60 s at each measuring point. The orthogonal components of the magnetic field were then interpolated to a 1x1 cm grid. Finally, the resulting field vector B_r at a given position was calculated based on the interpolated B_x, B_y and B_z components (). The magnetic flux densities were plotted in the figures as a function of the mattress positions. The grey scaling of all diagrams are identical; the higher the magnetic flux densities the darker the surface diagram will be.

Materials

Measurements of electromagnetic fields were performed with the following equipment regularly used in the neonatal intensive care unit:

1. Isolette Infant Incubator, model C 100, (Air-Shields, Hatboro, PA, USA). The air temperature of the incubator is microprocessor controlled with the aid of a heating element and a fan with the effect of 350 W. The unit has a control unit type C28-2E Series 00. Additional humidity control equipment from Air-Shields was fitted. Measurements as described above were made in the standard incubator as well as with the incubator modified by moving the control unit 25 and 50 cm respectively down from its original position under the mattress.

2. (a) Air-Shields Intensive Care System VHA overhead radiant warmer (680 W, Air-Shields, Hatboro, PA, USA). Measurements were performed at the patient level, approx. 70 cm from the radiant heater. (b) AMEDA radiant warmer (Type MN, 680 W, Ameda AG, Zug, Switzerland). Also in this case the measurements were performed at patient level approx. 70 cm from the radiant heater.

3. BiliBed photherapy unit (20 W fluorescent bulb, Medela AG, Baar, Switzerland). Measurements were made at the patient bed surface, directly above the light tube and power supply.

4. Kanmed heated water mattress (50 W, type 3785B, RD Teknomed, Sweden). Measurements were made on the mattress surface with the control unit located at the usual bed-end position (20-30 cm from the patient), as well as with the control unit placed at a distance 100 cm from its original position (away from the mattress).

Mapping of the electromagnetic fields was carried out with no measurable background field (i.e. below 0.01 μT). In the measuring position on the mattress the EMDEX II instrument was oriented in such a way that the x-direction always was in parallel with the long side of the mattress. The grid origin (i.e. co-ordinates 0;0) was defined as follows: for the incubator; the left nearest corner for a person standing in front of the service panel, for the radiant warmers; the left nearest corner when the supporting bracket for the warmer was oriented to the right, for the BiliBed; the left nearest corner when head position was oriented towards the right, and for the heated water mattress; the left nearest corner when the control unit was attached to the short side to the right.

Results

Incubator

The magnetic field strengths varied considerably in the incubator, both as a function of time as well as the position along the bed inside the incubator. Detailed data have been published by Aasen, et al.¹⁸ The mean broad band magnetic flux densities, with the main 50 Hz component, inside the incubators were typically within the range 0.2-1 μT, with maximum values around 1.5 μT, see Fig. 1 (the measured peak value at a single point was 1.8 μT being reduced in the grid averaging procedures when calculating the surface diagram).

Fig. 1 Time-averaged broadband (40 Hz - 800 Hz) magnetic flux density as a function of the position on the mattress. (Isolette Infant Incubator, model C100). The control unit was in its original position. Magnetic flux densities are given in μT and interpolated to a 1x1 cm grid from 5x5 cm measurements.

The magnetic fields originated mainly from the regulating units (with heating coil and fan) positioned below the incubator floor and the infant. The surface diagram in Fig. 1 shows that the magnetic fields at the mattress level are strongly varying in the intact incubator and with large field gradients. One can use the standard deviation values of the measured flux densities as a measure of the variation of the magnetic flux density across the mattress. This value (broadband data) was 0.34 μT for the intact incubator (control unit in its original position).

Fig. 1 also illustrates that the field level at the head position of the infant can vary considerably according to the orientation of the infant. As discussed later these fields levels can be of interest in consideration of possible effects of electromagnetic fields on melatonin concentrations. It can be observed that the magnetic fields in Fig. 1 are about 1 μT or 0.5 μT when the head is to the right or the left of the incubator respectively.

Lowering the control unit 25 or 50 cm below its original position reduced the magnetic flux densities at the mattress surface with a factor of about 5 and 15, respectively, as seen in Fig 2. As a consequence of the increased distance between the field sources and the mattress, the field gradients also decreased, as clearly indicated by the smoother surface diagrams. The variation of the magnetic fields across the mattress was in these positions 0.03 and 0.006 μT (values for standard deviation). Fig 3 shows the mean field levels (i.e. averaged over all positions on the mattress) as a function of the distance of the control unit from its original position.

Fig. 2 Time-averaged broadband (40 - 800 Hz) magnetic flux density as a function of the position on the mattress. (Isolette Infant Incubator, model C100). Measurements performed as in Fig. 1 but the control unit is now lowered 25 and 50 cm respectively, in (a) and (b).

Fig. 3 Mean broadband (40 - 800 Hz) magnetic flux density at the mattress level as a function of the vertical distance the control unit was moved (from its original position). (Isolette Infant Incubator, model C100). The flux densities in Fig. 1 and 2 have been averaged over all positions. Standard deviation is used to quantify variations of the flux density over the positions of the mattress.

Radiant warmers

Both types of radiant warmers investigated showed low flux densities Below the over-head radiant warmers the flux densities in the position of the child were always below 0.05 μT when the distance between the infant to the heating coils was approx. 70 cm.

BillBed unit

The flux densities on top of the "Bilibed" phototherapy unit were in the range 0.15-2.60 μT, with maximum values directly above the light-tube and power supply, see Fig. 4. In the figure a contour map has also been drawn to indicate the localisation of the top field values. The BiliBed unit has a light treatment area, indicated by broken lines in the figure. It is clearly seen that the highest magnetic field values are localised outside the treatment area and at the position of the head of the infant. The magnetic flux density level was about 2.5 μT at the head area.

Fig. 4 Broadband (40 - 800 Hz) magnetic flux densities as a function of the position across the Bilibed phototherapy unit. The light exposure area is indicated by the dotted lines, and the flux density diagram has been completed with a contour map to indicated that the highest flux levels are situated outside the treatment area, at the position of the head of the infant.

Water mattress

The heating-pad under the water-mattress produced no magnetic fields, in contrast to its control unit (with power supply). The control unit is generally located 20-30 cm from the infant. The flux density over the mattress was found to be between 0.09 and 2.3 μT (mean 0.73 μT) as shown in Fig. 5. Placing the control unit one metre away strongly attenuated the fields: the mean decreased to about 0.06 μT.

Fig. 5 Broadband (40 - 800 Hz) magnetic flux densities as a function of the position across the Kanmed waterbed when the control unit was located 20-30 cm away.

Discussion

As previously reported,¹⁸ the present magnetic flux densities are rather large compared to the magnetic flux levels in domestic homes. Karlsen et al.²⁰ performed spot measurements of the B-fields in Norwegian homes. Their findings varied from about 0.006 - 0.015 μT in bedrooms and living-rooms and 0.120 - 0.190 μT in kitchens. The largest measurements were taken in the heating season probably since electric heating is common in Norwegian residential houses. Hansson Mild et al.¹⁰ compared Swedish and Norwegian field levels and found mean flux densities of about 0.01 μT for living rooms and bedrooms and 0.02 μT for kitchens. The values were somewhat higher in Swedish residential houses, probably due to different wiring system in the two countries. The B-fields in the present incubator were a factor 100 higher than this, with a maximum value of 1.8 μT. Highest BiliBed and waterbed mattress values were 2.6 μT and 2.3 μT, respectively. For the sake of comparison, high-voltage transmission lines (about 300 kV) produce magnetic fields at the order of 1 μT at a distance of 0 - 50 m from the lines.²¹

Newborn, and particularly premature infants are still in a state of development, particularly with regards to important organs such as the central nervous system. The sensitivity to such fields might therefore be higher than for adults or children in general.

Compared to many industrial exposures, such as exposures in the aluminium works,²² where levels in the mT region (1 mT= 1000 μT) is not uncommon, the presently measured flux densities in the incubators are relatively small. However, exposures to magnetic fields in normal homes/occupations and also in most work places (power plants etc.) are not continuous throughout the day, as will usually be the case for infants treated in an intensive care unit.

The biological importance of the exposure time is not known, nor is the vulnerability of infants in this stage of development. Establishing possible limits for health effects on the infants is therefore difficult. However, measurements of the magnetic fields, both with respect to strength and frequency indicate possibilities of reducing the fields by reconstruction of the incubators, monitoring equipment etc. The present study thus shows clearly that a substantial reduction in field levels can be obtained by moving the source of the electromagnetic fields away from the infant. Thus, by moving the control unit 25 and 50 cm. away from the infant in the incubator resulted in a reduction of the fields to 1/5 and 1/15 strengths respectively. Similarly, although the heating element in the Kanmed heated waterbed does not create electromagnetic fields, the control unit, which is usually fixed to the side of the bed, caused substantial electromagnetic fields at the mattress surface. By moving the control unit 100 cm away from the bed, these fields were almost eliminated. In view of this simple strategy for reducing the electromagnetic fields to infants, the concept of the construction of the BiliBed phototherapy unit needs some considerations. The therapeutic effect of this unit is strongly connected to positioning of the infant directly above the fluorescent tube and electrical unit. If the BiliBed could be modified by increasing the distance to the infant, the light irradiance would, however, rapidly be reduced to non therapeutic levels. The magnetic fields caused by conventional overhead phototherapy equipment are usually somewhat lower, typically 0.5 μT,¹⁸ vs. the max value of 2.6 μT of the BiliBed unit. This is, therefore, an advantage of the conventional phototherapy units from a field point of view.

In the Results section we have several times indicated the magnetic field levels at the position of the head of the infants. This was to emphasise the fact that the head of the infants might be positioned at higher magnetic field intensities than the rest of the body. The possible magnetic field effects on the development of the central nervous system as well as on the melatonin production and the pineal functions^23-24 make this reasonable. However, the possible action of the magnetic fields needs to be clarified.

The biological significance of exposure of immature and sick newborn infants to increased levels of low frequency electromagnetic fields is not known. Of all the relevant risk factors that can affect these infants future lives, exposure to such fields is probably of lesser importance. However, until more knowledge has been obtained on the biological effects of low frequency electromagnetic fields, a reduction of these fields should be recommended, as part of a prudent avoidance strategy.

Acknowledgements

The authors thank the staff at the intensive care unit for their help during the registrations. The project was supported by the Norwegian Research Council (the EFFEKT-programme), and the Libero Diaper Fund (SABA-Mölnlycke AB).

References

1. Feychting M, Schulgen G, Olsen JH, Ahlbom A. Magnetic Fields and Childhood Cancer-A Pooled Analysis of Two Scandinavian Studies. Eur J Cancer 1995;314:2035-9.

2. Savitz DA, Wachtel H, Barnes FA, John EM, Tvrdik JG. Case-control study of childhood cancer and exposure to 60-Hz magnetic fields. Am J Epidemiol 1988;128:21-38.

3. Olsen J, Nielsen A, Schulgen G. Residence near high voltage facilities and risk of cancer in children. BMJ 1993;307:891-5.

4. Tynes T, Haldorsen T. Electromagnetic fields and cancer in children residing near Norwegian high-voltage power lines. Am J Epidemiol 1997;145:219-26.

5. Linet MS, Hatch EE, Kleinerman RA, et al. Residential exposure to magnetic fields and acute lymphoblastic leukemia in children. N Engl J Med 1997;337:1-7.

6. Coleman MP, Bell CMJ, Taylor H-L, Primic-Zakelj M. Leukemia and residence near electricity transmission equipment: A case-control study. Br J Cancer 1989;60:793-8.

7. Sandstrom M, Berglund A, Hansson Mild K. The office illness project in Northern Sweden - a study of offices with high or low prevalences of SBS: Electromagnetic fields in our indoor environment. Proc Indoor Air 1993;1:303-7.

8. Juutilainen J, Läärä E, Pukkala E. Incidence of leukemia and brain tumours in Finnish workers exposed to ELF magnetic fields. mt Arch Occup Environ Health 1990;62:289-93.

9. Sahl JD, Kelsh MA, Greenland S. Cohort and nested case-control studies of hematopoietic cancers and brain cancer among electric utility workers. Epidemiol 1993;4: 104-14.

10. Hansson Mild K, Sandström M, Johnsson A. Measured 50 Hz electric and magnetic fields in Swedish and Norwegian residential buildings. IEEE Transactions on Intrumentation and Measurement 1996;45:710-4.

11. Shaw GM, Croel L. Human adverse reproductive outcomes and electromagnetic field exposures: Review of epidemiologic studies. Environ Health Perspect 1993; 101(Suppl 4): 107-19.

12. Juutilainen J, Matilainen P, Saarikoski 5, Läärä E, Sakari S. Early Pregnancy Loss and Exposure to 50 Hz Magnetic Fields. Bioelectromagnetics 1993;14:229-36.

13. Savitz DA, John EM, Kleckner RC. Magnetic field exposure from electric appliances and childhood cancer. Am J Epidemiol 1990;131:763-73.

14. Savitz DA, Chen J. Parental occupation and childhood cancer: Review of epidemiologic studies. Environ Health Perspect 1990;88:325-37.

15. Anger G. Magnetfält från barnkuvöser och barnvärmebädder. SSI-report 94-02. Stockholm, Sweden. Statens Strdlskyddsinstitut. 1994.

16. Bearer CF. Electromagnetic fields and infant incubators. Arch Environ Health 1994;49:352-4.

17. Paul M, Hammond SK, Abdollahzadeh A. Power frequency magnetic field exposures among nurses in a neonatal intensive care unit and a normal newborn nursery. Bioelectromagnetics 1994;15:519-29.

18. Aasen SE, Johnsson A, Bratlid D, Christensen T. Fifty-Hertz magnetic field exposure of premature infants in a neonatal intensive care unit. Biol Neonate 1996;70:249-64.

19. Ramstad 5, Johnsson A, Bratlid D, Christensen T. Magnetic field reduction in an infant incubator. In : Johnsson A and Oftedal G, editors. 5th Nordic Workshop on Biological Effects of Low Frequency Electromagnetic Fields. 1997; StralevernRapport 6: 91-92. Norwegian Radiation Protection Authority, Østerås, Norway. ISSN 0804-4910.

20. Karlsen J, Johnsson A, Christensen T, Thommesen G. Maling av 50 Hz magnetfelter i noen norske husstander. Norwegian Radiation Protection Authority, Østerås, Norway. (with English summary). 515-report 1987:7. 1987.

21. Vistnes Al, Ramberg GB, Bjørnevik LR, Tynes T, Haldorsen T. Exposure of children to residential magnetic fields in Norway: Is proximity to power lines an adequate predictor of exposure 1997; 18:47-57.

22. Lödvsund P, Øberg PÅ, Nilsson SEG. ELF magnetic fields in electrosteel and welding industries. Radio Sci 1982;17:355-385.

23. Rosen LA, Barber I, Lyle DB. A 0.5 G, 60 Hz magnetic field suppresses meatonin production in pinealocytes. Bioelectromagnetics 1998;19: 123-7.

24. Reiter RJ. Alterations fo the circadian melatonin rhythm by the electromagnetic spectrum: A study in environmental toxicology. Regul Toxicol Pharamacol 1992;15:226-44.