CAES: Arsenic in Pressure Treated Wood

Arsenic in Pressure Treated Wood

 
David Stilwell
Department of Analytical Chemistry
The Connecticut Agricultural Experiment Station
123 Huntington St.
P.O. Box 1106
New Haven, CT 06504-1106

Voice: (203) 974-8457 Fax: (203) 974-8457

E-mail: David.Stilwell@po.state.ct.us

Arsenic is a component in the most prevalent wood preservative formulation in use. This formulation is known as CCA, which stands for chromated copper arsenic. Deep penetration of the water borne formulation is achieved by application under high pressure. The most common formulation results in a wood concentration of the Cr, Cu, and As of between 0.1-0.2%. Higher levels of arsenic from between 0.4-1.8% are employed for wood foundation and marine applications. Millions of board feet of CCA wood are produced each year, where it is used in the construction of decks, picnic tables, playground equipment, highway sound barriers, telephone poles and docks.

The benefits for using wood preservatives include:

Extending the life of the wood

Protecting the wood from harmful organisms such as fungi and termites.

Reducing the use of limited forest products, such as redwood.

Due to the massive amounts of CCA treated wood sold each year, dispersal of these additives from the wood could impact the environment. The potential toxicity of Cu, Cr, and As to humans, animals, and plants is well documented. Inorganic arsenic is a known human carcinogen. We have identified potential environmental concerns associated with the use of CCA wood:

Arsenic translocated to soil and water via;

leaching from wood

runoff from lumber yards

sawdust and physical wearing of the wood

Human exposure to arsenic (ingestion or inhalation);

dislodged from CCA wood surfaces (ingestion by hand to mouth)

during construction (inhalation of sawdust, and ingestion by hand to mouth)

by plant uptake (ingestion)

in soil (inhalation of dust and ingestion by hand to mouth

by burning the wood (inhalation. Generates toxic fumes- Do not burn)

Impact on beneficial marine organisms near docks built with CCA wood

Disposal of CCA wood

For this talk I will present a summary of our findings on (I) the amounts of Cu, Cr, and As in soils located under CCA treated wood structures, (II) on the determination of the amounts of arsenic dislodged from CCA treated wood surfaces, (III) Plant Uptake of arsenic, and (IV) Suggestions and Alternatives.

I.) Copper, Chromium, And Arsenic In Soils Under Decks Built From Pressure Treated Wood.

We conducted a field study to determine if there was any evidence that the CCA preservative could leach from the wood. For this field study the Cu, Cr, and As content in soils under decks built with CCA treated wood was determined. The decks ranged in age from 4 months to 15 years. Decks are ideal for the study of the effects of weathering due to rain and solar radiation on the wood. The wood is above ground and there are large amounts of horizontal surfaces so that any preservative leachate tends to flow directly to the soil below.

A total of 85 soil samples were collected from under a total of seven decks built with CCA pressure treated lumber. Only one of the decks had been coated with paint since construction. Control soils were acquired at a minimum distance of 5 meters from the decks. The samples were analyzed for Cu, Cr, and As by atomic spectroscopy.

We found that the average amounts of Cu, Cr, and As (mg/kg) in soils under decks were, 75 Cu, 43 Cr, and 76 As compared to 17 Cu, 20 Cr, and 4 As in control soils. The arsenic levels in the soils ranged from 3-350 mg/kg. Moreover, at each site, the average Cu, Cr, As content in the soil samples taken beneath the deck was elevated with respect to the average in the control soils, and these amounts in the soil tended to increase with deck age. Finally, the average amount of arsenic (76 mg/kg) exceeds the state limit of 10 mg/kg.

II.) Arsenic Dislodged from CCA Treated Wood Surfaces

A controversy exists on the extent of arsenic exposure due to physical contact with CCA wood surfaces. Such surfaces include playground equipment, decks and picnic tables built using CCA treated wood. Studies have shown that virtually no inorganic arsenic is absorbed through the skin, but is readily taken up by ingestion. Thus, the potential exposure is hand to mouth, and therefore, children are considered the most vulnerable to this potential risk. Our study attempts to investigate this vulnerability by analyzing the Cu, Cr, and As in wipe samples taken on a variety of CCA treated wood surfaces.

The wood surfaces were tested using polyester cloth material attached to an 8x13 cm wood block. Before use, the cloth materials were dampened with 1.5 times their weight with deionized water, and stored in polypropylene containers To minimize the effects of wood surface irregularities, the bottom of the block was cushioned with rubber, and sealed with polypropylene tape. Prior to sampling, a 1.25 kg weight was placed on top of the block. A wipe sample was taken by pulling the weighted assembly back and forth across 28-30 cm of the test surface a total of 5 times. The wipe was then placed back into the sample cup, and digested in 10% nitric acid solution for 2 hours at 60 degrees centigrade. The analyte concentrations were determined by atomic spectroscopy. By use of this sampling and analytical method, we have shown analyte recoveries of more than 90% using either dried CCA extract on glass surfaces or finely ground CCA wood powder. This method was designed to be consistent and reproducible for meaningful comparative measurements. The method was not designed to mimic hand to mouth exposure.

One series of CCA wood surfaces tested consisted of CCA treated wood purchased at lumber yards. For this study, seven sets of eight foot long CCA pressure treated boards were purchased from 3 lumber yards over a period of one year. Each set consisted of 3-4 boards, and the total number of boards surveyed was 20. Each board was cut into 1 or 2 foot coupons. Between 2 and 4 coupons from the interior portions of each board was tested. Three of the sets consisted of pine that was pressure treated with both CCA and a water repellent. These water repellent (WR) CCA boards are commonly used for decking (5/4 x 6 inches). The coupons were placed on racks outside, and a total of 336 wipe samples were taken on a periodic basis, for a period of up to two years.

We also surveyed CCA wood surfaces on wooden playscapes built using CCA treated wood from three municipal parks. A total of 45 wipe samples were taken from horizontal deck plank surfaces, and a total of 12 samples were taken from the vertical poles used to support the structures (when testing the vertical pole surfaces a hand was substituted for the block and weight).

The amounts of arsenic dislodged (g/100cm2) ranged from 7-122, and averaged 35 g/cm2 on the coupon surfaces, while the playscape surfaces ranged from 2-45, and averaged 8.8, and the pole surfaces ranged from 5-632, and averaged 35.

In all cases measurable amounts of arsenic were dislodged from the surfaces of these test coupons. The standard deviation from the average shows that there was high variability both within a particular set as well as between sets. There were no statistical differences between the amounts of arsenic dislodged from the surface of wood coupons containing both water repellents and CCA, and those containing CCA only. And there was no trend with respect to age on the amounts of arsenic dislodged from the test coupons, over the time period indicated. The levels of arsenic dislodged from the wood surfaces reported here are similar to those found by the Consumer Product Safety Commission study on CCA wood purchased from lumber yards.

The average amount of arsenic dislodged on the wood surfaces on the playscapes (8.8 g/100 cm2) was considerably less than the average reported for test coupons (35 g/ 100 cm2, Table 4). The reasons for the lower values on the surfaces of boards actually in use are not understood. The amounts of arsenic dislodged from these pole surfaces ranged from 5-632 g/100cm2 and averaged 105 g/100 cm2. Though these values were much higher than those observed on the horizontal surfaces, these results should be taken as indicative, as the testing method for the horizontal and vertical surfaces were not the same.

Using these numbers we can very roughly estimate the amount of arsenic that could be ingested. The estimate is as follows; assume a 1 to correspondence between wipe and hand, assume 50 cm2/hand, and an average ingestion of 0.4 handfuls/day. This works out to multiplying the results by 0.2 to obtain the estimated ingestion per day.

By using this estimate the values obtained (in g/day) are 7 (coupons), 2 (playscapes), and 21 (poles). The average amount of inorganic arsenic in the diet is 5 g/day, and that in water ranges from less than 5 to 100 g/day.

Finally, the effectiveness of coatings to form a surface barrier for arsenic was tested. For this study , the amounts of arsenic dislodged from CCA wood surfaces was determined before and after coating with a paint or stain. Four coatings were applied to four coupons each. The coatings were (1) polyurethane deck and porch enamel, (2) a latex acrylic solid color stain, (3) a spar varnish, and (4) a semi-transparent oil stain contain alkyl resins. The spar varnish is not recommended for use on foot-traffic areas. The amounts of arsenic dislodged from the surfaces was determined before, after, and up to 1 year after coating. Compared to the precoat values, there was more than a 95% reduction in the arsenic dislodged from the CCA wood surfaces coated with polyurethane, acrylic, or spar varnish. The percent reduction on surfaces coated with the oil based alkyl resin ranged from 80-97% and averaged 90%. It should be pointed out, however, that this test did not determine how well these coatings stood up to wear and tear. Information on this has recently appeared in Consumer Reports. We advise consulting with your paint dealer in order to determine which coating would most appropriate for a given use, such as high foot traffic areas.

(III.) Plant uptake of arsenic in romaine lettuce

Romaine lettuce was container grown in media to which either CCA sawdust or CCA wood blocks were added. After 26 days of growth, the lettuce was harvested, dried, and analyzed for arsenic. The lettuce grown alongside the CCA wood blocks contained 1.70.32 mg/kg arsenic (dry weight basis). Lettuce grown in media containing CCA sawdust, at a 32 and 480 mg/kg and arsenic level, contained 0.43 and 4.1 mg/kg arsenic respectively. Lettuce grown in control conditions contained less than 0.4 mg/kg. Converting these to g arsenic/ 50 g serving yields the following; <1 (control), 1 (grown in media containing 32 mg/kg arsenic), 10 (grown in media containing 480 mg/kg arsenic), and 4 (grown media alongside CCA wood blocks). These values compare to the dietary intake of inorganic arsenic of 4-12 g/day (all age groups).

(IV.) Suggestions and Alternatives

Coat CCA wood with paints or stains formulated for such use, and recoat as required.

Keep children and pets out of under-deck areas.

Follow safe handling and disposal guidelines when using CCA-wood

Consider use of alternative products on areas that may be contacted by children

Line interior of raised beds constructed using CCA wood with plastic.

(V.) More Information

Fact Sheet on the Proper Use and Disposal of Treated Lumber by the CT DEP
 
Fact Sheet on CCA wood by the CT DPH
 
Research on CCA by the Florida Center for Solid and Hazardous Waste Management




Content Last Modified on 6/1/2007 4:25:37 PM