RESEARCH

Prevention Measures

WHY MEASURE Pb DEPOSITION RATES OVER SHORT TIME PERIODS?

Mike van Alphen

Overview

Child Pb exposure has been a recognized issue in Port Pirie, South Australia, for 20 years [1]. Of the children from all parts of town aged under seven years in 1984[2], 96% exceeded a blood lead level of 10 µg/dl. In the high-risk areas of Port Pirie in 1996,[3] 75% of children under five were over 10 µg/dl. The program of the Port Pirie Environmental Health Center (EHC) plays a key role in the reduction of child blood lead levels in both high and low risk areas of Port Pirie[2,4].

Lead Smelting has been a dominant industry in the town for over 100 years and the current production rate exceeds 200,000 tones of Pb metal per year. The nearest residences are 750 m from the smelter center, and high risk areas for elevated child blood lead (PbB) are located in sectors associated with prevailing wind directions from the smelter.

Measuring Pb Deposition Rates Over Short Time Periods

Using collection devices having a large surface area, Pb deposition can be measured over hours [5] , not the more usual accumulation periods of ~ 30 days. Within 1000 m of the smelter Pb deposition can be measured using ½ m2 trays over 30 minutes if the wind exceeds some 3 to 4 m/s. In the backyards of houses, within 2.4 km of the smelter, Pb deposition has been measured in 1m2 trays in periods of six hours. Indoor sampling has used 1m2 trays over periods of ~12 hours. Quantification of Pb deposition in a smelter setting using ½m2 and 1m2 trays, accounting for field blank values, is satisfactory where > 5 to 20 micrograms of Pb can be collected. In low-Pb settings, field deposition of > 0.5 to 2 m g of Pb could be readily quantified from wipes of 1m2 trays [5].

Pb Deposition in Port Pirie

Smelter deposition: [1-3 hour sample intervals ½ m2 deposition trays]

At 500 m east of the smelter, a line of up to 11 deposition trays have been deployed at 150 m intervals.[6] A geometric mean Pb deposition rate averaged over the plume width, from 10 sampling events of ~ 19 mg/m2/day. The average Pb deposition rates measured ranged from ~ 6 to 180 mg/m2/day and increase with wind speed. Lead deposition is strongly dependant on the wind being from the central part of the smelter.

Backyard deposition: [6 hour sample intervals, 1m2 deposition trays]

Deposition trays were placed 5 m from the rear of up to nine vacant houses in high-risk areas of Port Pirie. Outdoor Pb deposition at up to 2.4 km from the smelter was dependent on the wind direction being stable and from the smelter. Higher wind speeds were encountered in this work ( >6m/s ) compared to the smelter study; yet sites not having downwind from the smelter encountered low Pb deposition rates. Contrary to previous views[7] , so called ‘city surface dusts’ contributed little to outdoor dust Pb deposition relative to winds from the smelter. At a house 2.4 km downwind of the smelter, where the wind speed exceeded 7 m/s, Pb deposition at 83 mg/m2/day was consistent with the above smelter survey results.

Indoor deposition: [37-69 day sample intervals, 0.137m2 alfoil collectors]

Pb deposition in vacant houses in high risk areas having windows and doors closed is significantly lower than when windows and doors are open.[8] Deposition measured at floor level can reach 1000*µg Pb /m2 in one week in an open house condition but take ten weeks to reach this level in a closed house condition. With windows and doors closed, there is ongoing recontamination of a vacant house, a function of the inherently poorly sealed nature of older houses and perhaps sinks of Pb dust in ceiling spaces and wall cavities. Higher Pb loadings at entryways and in rooms facing the smelter and the dominance of new Pb entering the house via openings are among the key findings of this work.

Indoor deposition: [~ 12 hour sample intervals, 1m2 deposition trays]

Two sampling exercises using cleaned vacant houses with windows open 150 mm have been conducted overnight during or after rainfall such that outdoor surface dusts can be eliminated as a source of dust Pb. When the wind direction is from the smelter with windows open, deposition of 1000* µg Pb /m2 can occur in periods of some 12 to 24 hours. Spatial gradients in Pb deposition can be demonstrated between rooms over short time periods within houses.

The Pattern of Recontamination in Time and Space.

The recontamination of house interiors by dust Pb in Port Pirie is not a constant process in time or space. The gap between a fly / mosquito wire screen and the window is a classic sheltered location where dust Pb accumulation occurs. As with verandahs, narrow alleyways* and ceiling spaces, deposition appears to occur in confined or partly enclosed space, and given that these sites are (often) sheltered from rain, dust Pb accumulates. Where wind speeds drop in sheltered settings and the vertical component of wind turbulence fails to suspend particles, deposition appears to occur. A house interior is also such a sheltered space. With potent outdoor air Pb sources, high levels of Pb deposition are found on the Pb source side of the house adjacent to dust entry points, and room to room differences can be pronounced. Outdoors, gradients in Pb deposition rates are likely to occur with high levels in semi-enclosed or sheltered locations.

At a distance of some 700-1000 m from the center of the Pb smelter, the plume of depositing Pb is some 600 m wide. Small angular-segments of residential areas are impacted upon by the smelter emission plume at any time. Discrete Pb deposition events occur when wind speeds exceed 7 to 10 m/s and the wind direction remains fixed over a narrow segment of a residential area for more than 6 to 12 hours. Where the wind direction swings around freely, depositing Pb is dispersed more widely and particular Pb deposition events are less easily detected. A spectrum exists from high Pb deposition rate extended-duration but low-frequency events to lower-impact high-frequency events.

What Sort of Secondary Intervention?

Ongoing Pb emissions produce accumulations of Pb in the soils and dusts of residential settings, generating more intractable problems over time. The primary focus of intervention should be the control of Pb emission at the source. There are however secondary measures that may be considered. These include behavior modification and building modifications that include dust proofing.

Practical as well as readily understandable advice to assist in the reduction of child Pb exposure[2 ] can include; using and cleaning door mats, wet mopping and damp dusting, not disturbing paint Pb when children and pregnant women are present in houses, not drinking rainwater, children eating at the table and not snacking on the floor, washing hands before eating, washing fruit and vegetables, etc. Some of these messages are universal and some of this advice will be specific to a location such as Port Pirie. Other advice may be case specific. Observations of dust entry through cornice cracks, and of Pb rich dust accumulations in the ceiling space (many centimeters thick) were the basis for interventions such as cleaning ceiling spaces and sealing gaps in room linings in Port Pirie in the 1980’s. Advice relating to sealing gaps to reduce dust entry into houses was closely monitored. The tracking of soil, mud and dust into the home suggests the use of doormats.

There is potential for additional advice or secondary intervention to be considered, justified by short-term deposition sampling data and dust accumulation observations. Such intervention depends on the existing air exchange rate of sealed houses, whether house ventilation can be controlled by occupants, and whether adequate cleaning methods are or can be employed. The insights provided here are based in part from investigations in vacant houses, and clearly more information is required on the short-term deposition rates in occupied homes.

Recommendations that could be considered and tested by short-term Pb deposition and other sampling methods include:

  1. Close windows when high speed winds are from a Pb emission source direction
  2. Regularly hose down all fly wire screens and outdoor window sills
  3. Ensure that windowsills are clean or are cleaned when opening windows
  4. Place a child’s bed away from windows and other dust Pb entry points
  5. Relocate a child’s bedroom within a house to a lower dust and Pb deposition setting
  6. Minimize the use of door access on the Pb source side of a house, and don’t use external doorways if they happen to be in a child’s bedroom
  7. Clean areas such as entryways, window-wells, window-ledges and verandahs where Pb and dust readily accumulate more frequently
  8. Not drying clothing or airing bedding and soft furnishing materials outdoors when winds are blowing from Pb source directions
  9. Store infant’s prams and pushers in a dust free setting
  10. Don’t use soft furnishings as outdoor furniture on verandahs
  11. Time additional household cleaning to occur after high wind speed events from the Pb source direction

Detailed studies can be conducted to evaluate the above advice and validate strategies for reducing surface dust Pb loadings and reducing PbB. Aspects of the above (points 2, 4, 5, 6, 9 and 10) have been advocated in Port Pirie on a general basis, and on a case by case basis to residents for over the last 5-10 years. Visible surface dust accumulations can occur over short periods of time in Port Pirie, and the universal message of the EHC ‘...where there is dust there is Pb’ is also straightforward. The advice just listed here can be readily interpreted and is sensible but some of it may not be universal or practical. Actions based on interpreting wind direction may be problematic. Wind direction is easily observed in a sparsely settled residential setting while in an ‘urban canyon’ setting or hilly topography, wind may be more turbulent with direction not so predictable, constant or easily observed. Some of the advice listed has little cost and shows prospects of success without causing harm, while others warrant critical evaluation. Air-tight houses need windows to be opened or else indoor air quality issues[9] arise. Safe ventilation is an issue where potent outdoor Pb sources exist; additional investigation on the provision of safe ventilation in houses is warranted.

More sophisticated physical building modifications are also interventions that can be considered and tested based on the use of short-term Pb deposition measurement methods.

Conclusions

It is hypothesized that near point and linear Pb emission sources, dust Pb deposition in residential settings would not be constant when the sample interval is limited to periods of hours or days. Vehicle emission Pb, battery factories, secondary smelters, kilns burning sump oil and non-ferrous foundries etc. may also be associated with Pb deposition regimes that have temporal and spatial variability in residential settings. Some of that variability relates to the amount of Pb being emitted. The macro scale determinants of Pb deposition within residential settings may be building design, building orientation, source-receptor distance, any wind barriers, prevailing wind speed and wind direction. These factors and individual house characteristics can be evaluated in time and space by short-term deposition samples.

The Pb contamination of houses in Port Pirie is determined by wind speed and direction from the smelter, and significant Pb deposition on surfaces inside well-ventilated houses can occur in periods of hours. This contamination has an event based component not seen if one collects samples from the home setting that are long-term integrators of dust Pb deposition such as carpet and floor Pb loading measurements. Floor Pb loading measurements at weekly or monthly intervals will indicate an average Pb accumulation rate (± seasonal variation), that is a function of the averaging of numerous cyclical cleaning, redistribution and deposition events.

If child Pb exposure should be avoided pre-natally, at birth, at week one, month one, or in early stages, there should be a focus on avoiding any short-term accumulation of Pb in a child’s home setting. If the recontamination of a child’s home setting can be substantially determined by particular conditions of wind speed and direction over short periods of time, there may be opportunities to intervene to limit exposure or at least better understand exposure*. Similarly if there are known spatial gradients in Pb deposition rates in the home setting, interventions can be targetted. The means of investigating recontamination of the home setting include both long-term and short-term Pb deposition measurement strategies. Deposition rate measurement in residential settings in India may be a means of developing a universal measure of recontamination rates in houses where the use of ‘floor Pb loading’ analysis methods are fraught with difficulties caused by floor surfacing with wide ranges of materials.

The significance of methods for the measurement of Pb deposition rates over periods of the order of hours include:

    • The ability to use the wind trajectory to link an emission source with real-time deposition in homes
    • Understanding events that dominate the contamination of the residential setting and incrementally determine the accumulation of Pb in the home, and hence incrementally impact on child Pb exposure
    • The ability to determine whether interventions have measurable benefits.

References

[1] McMichael, A.J., Baghurst, P.A., Robertson, E.F., Vimpani, G.V. and Wigg, N.R., (1985) The Port Pirie Cohort Study: Blood lead concentrations in early childhood. The Medical Journal of Australia 143 pp.499-503.

[2] Port Pirie Environmental Health Center (1999) http://www.health.sa.gov.au/pehs/branches/branch-ptpirie.htm

[3] Maynard, E.J., Calder, I.C., and Phipps, C.V., (1993) The Port Pirie Lead Implementation Program. Environmental Health Branch. South Australian Health Commission. Adelaide.

[4] EHC (1997) Pea-bee’s Update. Parents of Port Pirie once again take a bow!!! Information pamphlet. Port Pirie.

[5] van Alphen M, Maynard EJ, (1998) A method for measuring metal deposition rates over short time periods. Proceedings of the 14th International Clean air and environment conference. Melbourne 18-22 October 1998.

[6] van Alphen M, ( submitted Jan 1999) Atmospheric heavy metal deposition plumes adjacent to a primary lead-zinc smelter. Contact MvA for further details vanalphen.mike@health.sa.gov.au. or mva@camtech.net.au

[7] Body, P.E., Inglis, G.R., and Mulcahy, D.E., (1988) Lead contamination in Port Pirie South Australia. South Australian Department of Environment and Planning Report 101 April 1988, Adelaide.

[8] Kutlaca, A., (1998) Mechanisms of Entry of Lead-Bearing Dusts into Houses in Port Pirie. PhD Thesis University of Adelaide.

[9] USEPA (1995) The Inside Story: A guide to Indoor Air Quality. EPA Document # 4002-k-93-007.

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