OFFICE OF RESPONSE AND RESTORATION ! EMERGENCY RESPONSE DIVISION

Deepwater Horizon Oil: 

Characteristics and Concerns

Oil spilled from the Deepwater Horizon wellhead has been accumulating in the environment of the Gulf of 

Mexico since April 20. This document describes the potential forms the oil may take over time as it weathers 

and degrades; its chemical makeup; its hazards to humans and the environment, especially on beaches and 

land; and the cleanup protocols most likely to be effective in removing it. Table 1 (next page) summarizes this 

information.

Chemical nature of the oil

Mississippi Canyon Block 252 (MS252) oil is a South Louisiana sweet crude oil (crude oil is termed sweet if it is

low in sulfur). Like all crude oils, MS252 oil is a complex mixture of thousands of chemical compounds. Of 

those thousands of compounds, several classes are of special importance in considering persistence and 

toxicity in the environment. Compared with other crude oils, MS252 oil is relatively high in alkanes. Because 

alkanes are made up of single-bonded carbon chains that microorganisms can readily use as a food source, 

MS252 oil is likely to biodegrade more readily than crude oils generally. MS252 oil is less toxic than crude oils 

generally because it is relatively much lower in polyaromatic hydrocarbons (PAHs). PAHs are highly toxic 

chemicals that tend to persist in the environment for long periods of time, especially if the spilled oil 

penetrates into the substrate on beaches or shorelines. Like all crude oils, MS252 oil contains volatile organic 

compounds (VOCs) such as benzene, toluene, and xylene. Some VOCs are acutely toxic but because they 

evaporate readily, they are generally a concern only when oil is fresh.

Weathering processes

As MS252 oil reaches the surface and spreads out across the water, its lighter components, including VOCs,

soon evaporate, leaving heavier components behind. Some of the remaining MS252 oil will become sheen, a 

very thin layer of floating oil (less than 0.0002 inches or 0.005 mm) that can be transparent, grey, silver, or

rainbow-colored. MS252 oil will also mix with water to form a sticky, pudding-like water-in-oil emulsion, or 

mousse, typically brown, reddish, or orange in color. Typically, crude oil emulsifies on the sea surface as winds 

and waves mix it with water, but MS252 oil also appears to be incorporating water as it rises to the surface 

through 5,000 feet of water. Winds and waves tear oil and mousse patches into smaller pieces, eventually 

producing tarballs. MS252 tarballs typically are in the form of small, hard, black pellets. Tarballs can be very 

persistent in the marine environment.

Hazards and cleanup methods

MS252 oil poses physical and chemical hazards to people and the environment. On beaches and land, fresh 

or emulsified oil can smother plants, birds, and animals and can create a skin contact hazard for people. 

VOCs can pose an inhalation hazard to people working near fresh oil, sheen, or mousse. PAHs can cause a 

persistent environmental hazard if the oil penetrates substrates or is not removed from beaches. A variety of 

standard cleanup protocols are appropriate for removing MS252 oil from beaches, shorelines, and adjacent 

waters (see Table 1).
TABLE 1. Forms of Mississippi Canyon 252 oil likely to be found on Gulf Coast beaches and shorelines: characteristics, hazards, and cleanup 

protocols likely to be most effective.

OIL TYPE DESCRIPTION CHEMICAL CHARACTERISTICS ENVIRONMENTAL & HEALTH HAZARDS CLEANUP PROTOCOLSFresh oil Black or dark brown, thick, sticky liquid 

with petroleum odor. On open water,floats and spreads quickly. In intertidal when washed by tidesink. On beach, may release sheen zone, may pick up silt and sediment and 

s or waves. May 

penetrate beach substrate.

compoundsMixture of 1,000s of chemical 

. Some compounds

persist for years.evaporate quickly, others can 

High in 

MC252alkanes, VOCs; flammable. 

oil is low in sulfur and

relatively low in PAHs. 

Smothering of plants, birds, and animalsis major concern. PAHs are toxic to fish eggs, larvae. Human health hazard viainhalation, skin contact, and (less likely)ingestion. VOCs are acutely toxic.

On water: disperse, skim, On land:burn

sorbent pads, 

with watmanual recovery, flushing 

er, possible use of 

agentschemical shoreline cleaning 

mixtureor oil/water (emulsified oilMousse 

)

Brown, rust, or orange in color. Pudding‐like, sticky. May de‐emulsify which could penetrate substrateon beach in hot sun, releasing fresh oil 

. If not

de‐emulsified, unlikely to penetrate substrate.  Little odor.

Consists of various percentages of water in emulsion with fairly fresh oil. Reduced VOC content. ignitability & biodegradabilityWater content reduces

.

Sticky, can smother plants, birds, and animals. Human health hazard via inhalation, skin contact, and (less likely)ingestion. VOCs are acutely toxic.

On water: skimOn land: sorbent pads, manual recovery

Sheen Very thin layer of fresh oil. May be 

transparent, rainbow, grey, or silvery in color. May be associated with adjacent fresh oil or mousse.

Most VOCs evaporated. Low levels of PAHs may be present. Heavier sheens may 

Light sheens will degrade quickly.  

concentrate on 

shorelines. Human health hazard from skin contact, inhalation, and (less likely)ingestion.

Light sheens very difficult to recover. Heavier sheens may be picked up with sorbent boom or sorbent pads.

Tarballs MC252 oil will weather to form small, 

hard, floating, black pellets or chunks of oil. May be highly persistent and travel long distances. On beach, may soften in hot sun. In intertidal waters, may pick up sediment or silt and sink.

Asphalt‐like tar. VOCsevaporated. PAHs still present. birds and animals. Prolonged s

May soften in hot sun, posing hazard to 

kin 

contact may cause allergic reaction or rash. May be a concern for recreational shorelines.

On water: snare boomOn land: snare boom, manual removal, beach cleaning machinery 

(floatingBurn residue

or on 

land)

Brittle, hard, asphalt‐like, typically mixed with unburned fresh oil. May be mistaken for tarballs. 

Asphalt‐like tar. VOCs evaporated. PAHs still present. 

Little or no acute aquatic toxicity. Very offshore, controlled burning is conducted well localized smothering impacts. Because 

human health concern on land

is minimal.

from Collect residue manually

water or land

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