Understanding Blood Alcohol Content (BAC)

When you consume an alcoholic drink, within minutes the human body can absorb the alcohol contained in the beverage and transport it to all parts of the body through the bloodstream.

Once it reaches the central nervous system, alcohol negatively affects brain function, muscles, and vision, drastically reducing all abilities to drive vehicles safely.

Blood Alcohol Concentration (BAC)

Blood alcohol concentration (BAC) is assessed based on the indicator number of grams of alcohol present in each litre of blood (g/l).

Therefore, a driver with a BAC of 1.0g/l has one gram of alcohol in each litre of their blood.

During enforcement actions, police authorities may use approved devices (commonly known as breathalysers) that, based on the alcohol content in exhaled air, indicate the value of blood alcohol concentration.

Blood alcohol concentration can also be obtained through blood analysis, which must be performed at an official health establishment (as happens in cases requiring a counter-proof request).

* Applies to drivers on probation, drivers of emergency or urgent service vehicles, collective transport of children and young people up to 16 years, taxis, heavy passenger or goods vehicles, or transport of dangerous goods.

Factors Affecting Blood Alcohol Concentration

As someone consumes an alcoholic beverage, without realising it, their intellectual and muscular performance becomes increasingly reduced, while they acquire a euphoric state that gives them a false and dangerous sense of security - a situation that only fades after a long and reasonable time.

Alcoholic beverages produce a BAC in individuals that is higher when:

• The drink has higher alcohol content
• Greater quantity is consumed
• Consumption happens more quickly
• The person weighs less
• The person has an empty stomach

Main Effects of Alcohol on Driving

Whether consumed as fermented or distilled beverages, alcohol has effects (almost immediately after consumption) on the driver, even when absorbed in small quantities.

• Decreased concentration
• Reduced visual acuity (object contours lose definition)
• Narrowed visual field (which can lead to "tunnel vision")
• Impaired judgement of distances and speeds
• Increased recovery time after glare
• Hearing impairment
• Increased reaction time
• Reduced reflexes (movements become slow, sometimes abrupt, in any case imprecise)
• Creation of a false state of euphoria and overestimation of abilities
• Increased risk of accident

Driving Under Alcohol Influence and Road Accident Risk

According to information provided by the Directorate-General for Traffic, the risk of involvement in a fatal accident increases rapidly as the concentration of alcohol in the blood becomes higher.

Note: Blood alcohol concentration is not proportional to accident risk - the relationship is exponential rather than linear.

Body's Process of Eliminating Alcohol

The metabolic system of the human body works through a natural and slow process to eliminate alcohol contained in the blood through the liver (90%), urine, exhaled air, and perspiration (10%).

The liver breaks down and removes alcohol from the blood at an average rate of 0.10g/l per hour.

Therefore, an individual with a BAC of 1.2g/l will have naturally eliminated all alcohol from their body after 12 hours, provided that no alcoholic beverage is consumed during this period.

This process cannot be sped up by any means, and it is not possible to eliminate the effects of alcohol.

However, there are substances and factors that disrupt this elimination, namely by delaying the normal functions of the liver, or enhancing its harmful effect, such as coffee, tea, tobacco, certain medications, and fatigue.

Alcohol and Medications

Drivers who take medications or similar chemical products to reduce depression or control nervous tension may lose their ability to drive for some time and risk causing a road accident if they do not refrain from driving while under the effects of these products.

Psychoactive Substances

A large portion of road accidents result not only from driving under the influence of alcohol but also under the effect produced by psychoactive substances, whose enforcement and sanctions are strictly defined by law.

These products, commonly referred to as "drugs," are likely to modify the mental and motor faculties of those who consume them, producing behaviours with effects similar to those already listed for alcohol consumption.

Psychoactive substances physically and mentally weaken their regular consumers, potentially leading to progressive dependence that may temporarily or permanently incapacitate them for driving.

Main Effects of Psychoactive Substances on Driving

As with other drugs, the action of these substances varies considerably, depending on the individual, environment, and circumstances. However, their most common effects can vary, depending on the type of substances consumed, between:

Euphoric Sensation Manifested as:

• Nervous excitement
• Insomnia
• Talkativeness
• Increased confidence and self-satisfaction
• Agitation
• In some cases, aggressiveness, fatigue, and hyperactivity

Sensation of Tranquility:

• Slightly decreasing blood pressure and heart rate
• Altering consciousness
• Occasionally causing euphoria

Depending on the quantities, they can reduce reflexes, cause weakness, accelerate heart rate (pulse), dilate pupils, and cause slow breathing, which can lead to a coma state and death.

• Depressive or stimulating effects
• Hallucinogenic effects
• Slowed reactions
• False sense of security and driving ability
• Euphoria

Legal Framework

According to the Highway Code, it is prohibited to drive under the influence of alcohol. A driver is considered under the influence of alcohol if they present a BAC equal to or greater than 0.5g/l or if, after examination, they are considered as such in a medical report.

The conversion of values of alcohol content in exhaled air (BrAC) to blood alcohol content (BAC) is based on the principle that 1mg of alcohol per litre of exhaled air is equivalent to 2.3g of alcohol per litre of blood.

Prevention of Resuming Driving

A driver who presents a BAC equal to or greater than 0.5g/l will be prevented by the police officer from resuming driving for a period of 12 hours, so that their body can eliminate the alcohol contained in the blood.

For this purpose, the authority officer proceeds to immobilise the vehicle and, if necessary, remove it to a car park or appropriate location, arranging for the transportation of the remaining vehicle occupants, with expenses borne by the driver.

This procedure may not take place if another person, with the consent of the driver or vehicle owner, proposes to drive the vehicle, provided they present a negative result in the BAC test.