National Radiological Protection Board

Particle Deposition in the Vicinity of Power Lines and Possible Effects on Health:

Report of an independent Advisory Group on Non-ionising Radiation and its Ad Hoc Group on Corona Ions

Overall summary and conclusions

Production of corona ions

Whenever high voltages are present in electrical systems there is the possibility that the high electric fields that exist close to conductors may cause electrical breakdown of the surrounding air; an effect known as corona discharge. Corona discharges often occur from electrodes that are at a potential of several thousand volts with respect to ground. DC and AC high voltage transmission lines are examples of this, but are of course designed not to operate under corona discharge conditions because this would result in loss of power, and also produce noise that would cause complaints. However, small local intensification of the electric field at the conductor surfaces can arise at dust and dirt accumulations or at water drops causing corona discharges to occur. In addition, some power lines are operated at higher voltages than their design values.

As a consequence of corona discharges, high voltage AC power lines may produce clouds of negative or positive ions that are readily blown downwind. An increase of charge density downwind of power lines is well established and can be measured at distances up to several kilometres. The ion clouds charge pollutant particles that pass through them. These particles will already carry some charge because of the naturally occurring ions that exist in the atmosphere but it seems likely that in some regions this will be increased even at ground level as a result of corona discharge. Calculating this increase as a function of particle size is possible but only if a number of simplifying assumptions are made. The effects indoors, where the majority of people spend most of their time, are probably somewhat less than outdoors – for example, because of deposition of corona ions on the surfaces of small apertures through which some air enters buildings. The presence of corona ions could influence the uptake of pollutants by increasing their deposition in the lung or on the skin.

Inhalation of pollutant particles
People may be exposed to these more highly charged pollutant particles and the effect of electrostatic charge on increasing respiratory tract deposition has been recognised for some time. The deposition of particles in the human respiratory tract has been studied extensively both experimentally and theoretically. Current models such as the ICRP Human Respiratory Tract Model (HRTM) enable the fraction of inhaled particles that is deposited in each region of the respiratory tract to be calculated, according to the size distribution of the inhaled particles and the breathing pattern and age of the subject.

There have been a number of experimental and theoretical studies of the effect of electrostatic charge on deposition in the respiratory tract. Increased deposition results from image charge forces. The effect increases with increasing number of units of charge on a particle, but decreases with increasing particle size. There is consensus that for particles larger than about 0.3 µm, charge is unlikely to have a significant effect on lung deposition. However, for smaller particles, there are no human in vivo data available. Measurements from one study using a model of human bronchial airways suggest that at about 0.1 µm a single unit of charge increases lung deposition by a factor of about three, which is more than predicted by current theory. Hence there does not seem to be a suitably validated model for predicting the effect of charge on deposition of submicron particles.

The HRTM does not address explicitly the effects of electrostatic charge. It has, however, been used in this report to assess the potential for charge to increase lung deposition. This was done by comparing the number of particles that enter the lungs with the number that deposit there in the absence of any charge effects. For very small (less than about 0.01 µm) and large particles (greater than about 5 µm), lung deposition is limited by deposition in the nose and upper airways (the extrathoracic airways, ET): most particles that reach the lungs deposit anyway. The effect of charge would therefore be to increase ET deposition and reduce lung deposition. In the size range about 0.1–1 µm, where lung deposition is normally low (about 10%), there is potential to increase lung deposition by a large factor, a theoretical maximum of about three to ten, depending on particle size. This size range does, however, correspond to a major fraction of the normal ambient atmospheric particle distribution. In any practical situation the effect of charge on deposition of a pollutant will be less than this theoretical maximum because of factors including the extent to which the pollutant particles are charged, the size distribution of the pollutant particles, hygroscopic growth, and increased deposition in the ET airways. Thus, as noted above, particles larger than 0.3 µm diameter are unlikely to carry a sufficient number of units of charge for lung deposition to be affected.

The effect on exposure of individuals will be lower still because of their ‘occupancy’ factor: the fraction of the time to which they are exposed to particles charged by corona ions. Henshaw and Fews (2001) have calculated that people downwind of power lines in corona might have 20%–60% more particles deposited in their lungs than those upwind. This estimate is for people exposed out of doors to pollutant particles which originate out of doors. When outdoor air enters houses, many of the pollutant particles will be carried with it (Liu and Nazaroff, 2003), so a similar effect would be expected indoors. The effects of corona ions on lung deposition of particles which originate indoors will be substantially less. There are substantial difficulties in the way of modelling such effects, making all such estimates very uncertain. Furthermore, since wind directions vary, the excess for any one group of people would be lower, but more groups will be affected, than if the wind direction was constant.

The increase in pollutant deposition in the lungs seems likely to be highest in areas of the country downwind of power lines where there are high levels of airborne particulate pollution and also where the power lines are continuously in corona. The latter is most likely where the power lines were designed for a lower voltage and have not been upgraded. Because of the high rate of production of corona ions in such situations, it seems likely that there will be a significant increase in charge per particle, even when the particle concentration is high.

The information reviewed suggests that some increase in lung deposition of pollutant particles seems likely as a result of charging by corona ions. Even if the effect of the corona ions were to cause all the particles to be deposited, the increase in lung deposition cannot be more than a factor of ten. In practice, though, the increase seems likely to be appreciably less and it is noted that Henshaw and Fews (2001) estimated it to be 20%–60%. Such estimates are, however, inherently imprecise since they depend on the use of an approximate model and on assumptions about the experimental conditions (the distributions of particle size and charge) which are not well known and not readily obtainable. The effects of external electric fields on deposition of particles in the respiratory tract, if any, are likely to be very small (paragraph 126).

Deposition on the skin
The additional charges on particles downwind of power lines could also lead to increased deposition on exposed skin. However, any increase in deposition is likely to be much smaller than increases caused by wind.

There is experimental evidence (Fews et al, 1999a) supported by theoretical analysis that the deposition of both radon decay products and chemical pollutants on surfaces are somewhat larger under power lines. The increase is considered to be around a factor of 2.4 for radon decay products and to be still significant, around 1.2, for chemical pollutants. This is attributed to the increase in deposition of the naturally charged particles produced by the oscillating electric fields together with turbulent air flow over the skin. The electric fields are screened by the walls and roofs of buildings. Hence any significant increase of deposition would only occur outdoors. The deposition of radon decay products would vary much less from place to place than that of chemical pollutants, whose deposition would be greater in towns, near industrial sources and next to major roads.

There are different views of the extent of increased deposition on exposed skin under power lines. Thus Swanson and Jeffers (2002) accepted that increased deposition of radon decay products would occur. However, they attributed the increased deposition of larger particles observed experimentally and predicted theoretically to the design of the experiments and to the parametric values and analytical expressions used by Fews et al (1999a).
These disparate views about changes in skin deposition under power lines cannot be resolved without further experimental measurements. It is possible that the differences in the theoretical analysis might be reduced by further work. However, the physical situation is very complicated and it seems unlikely that it can be modelled with sufficient accuracy to provide reliable information in the foreseeable future.
Implications for health
The main health hazards of airborne particulate pollutants are cardiorespiratory disease and lung cancer. There is strong evidence that the risk of cardiorespiratory disease is increased by inhalation of particles generated outdoors, mainly from motor vehicle exhaust, and of environmental tobacco smoke produced within buildings. The risk of lung cancer is increased by particulate pollution in outdoor air, and by radon decay products and environmental tobacco smoke in buildings. Any health risks from the deposition of environmental particulate air pollutants on the skin appear to be negligible.

The potential impact of corona ions on health will depend on the extent to which they increase the dose of relevant pollutants to target tissues in the body. It is not possible to estimate the impact precisely, because of uncertainties about:

(a) the extent to which corona effects increase the charge on particles of different sizes, particularly within buildings,

(b) the exact impact of this charging on the deposition of particles in the lungs and other parts of the respiratory tract,

(c) the dose–response relation for adverse health outcomes in relation to different size fractions of particle.

However, it seems unlikely that corona ions would have more than a small effect on the long-term health risks associated with particulate air pollutants, even in the individuals who are most affected. In public health terms, the proportionate impact will be even lower because only a small fraction of the general population live or work close to sources of corona ions.

RECOMMENDATIONS FOR FURTHER RESEARCH
The possible implications for health of the mechanisms discussed in this report do not provide a strong case for further research in this area. It is concluded, therefore, that it is not appropriate for an epidemiological study to be carried out. If, however, it were felt to be desirable to reduce some of the uncertainties in the analysis of the mechanisms for increased deposition, this could be done through the following studies.

(a) Experimental study of the charge and size distributions of airborne particles upwind and downwind of power lines – the technique recently developed by Fews et al (2003) may be of value in such a study.

(b) Studies of deposition of charged particles under power lines – any such study should include laboratory and outdoor studies to allow investigation to be made of the dependence on size and air flow.

(c) Experimental study of the effect of electrostatic charge on lung deposition for particles in the size range 0.005–1 µm – the study should include in vivo measurements.

(d) Development of a theoretical model to calculate the effect of electrostatic charge on lung deposition for particles in the size range 0.005–1 µm – any such study should be validated by comparison with the results of the experimental study.

Link to explanation of EMF fields.  Small flash movie.