Comparing Shopping Bags: A Practical LCA Revision Notes | GCSE AQA Higher Chemistry

1. The Shopping Bag Dilemma

You're at the checkout. The cashier asks: "Plastic or paper bag?" Most people think paper is better for the environment. But what if that's wrong?

This question isn't as simple as it seems. To find the real answer, we need to look at the entire life story of each bag - from the raw materials used to make them, right through to what happens when we throw them away. This complete picture is called a Life Cycle Assessment.

LCA considers four main stages: raw material extraction, manufacturing, use phase, and end-of-life disposal. Each stage has environmental costs that we need to measure and compare.

2. Breaking Down the Life Cycle

Let's trace the journey of both plastic and paper bags through each stage of their lives. You'll notice some surprising differences.

Stage 1: Raw MaterialsPlastic bags start as crude oil, which must be extracted from underground. Paper bags begin as trees, which need to be cut down from forests. Both processes have environmental impacts, but they're very different.

Stage 2: ManufacturingMaking plastic bags requires heating oil to high temperatures and using chemical processes. Paper bag production involves pulping wood, bleaching, and pressing - all of which need lots of water and energy.

Stage 3: Transportation and UseBoth bags need to be transported to shops. Here's where weight matters - paper bags are much heavier than plastic ones, so they need more fuel to transport.

Stage 4: End of LifeThis is where most people focus their thinking. Paper bags biodegrade naturally, while plastic bags can persist in the environment for hundreds of years if not recycled properly.

3. Comparing Environmental Impacts

Now comes the tricky part - measuring and comparing impacts. Environmental scientists look at several key factors when conducting an LCA comparison.

Energy Consumption: How much energy is needed to make each bag? Paper bag production typically requires approximately 3.4 times more energy than plastic bag production. This is because making paper involves energy-intensive processes like pulping and drying.

Water Usage: Paper bag manufacturing uses approximately 17 times more water than plastic bag production. The pulping process requires enormous amounts of water to separate wood fibres.

Air Pollution: Paper bag production generates 3.3 times more air pollutants than plastic bag production, mainly from the chemical processes used in pulping.

Solid Waste: Manufacturing paper bags produces approximately 4.8 times more solid waste compared to plastic bags.

These numbers might surprise you. But remember - we're not finished yet. We still need to consider what happens at the end of each bag's life.

4. The End-of-Life Reality Check

Here's where paper bags seem to have a clear advantage - they biodegrade naturally while plastic bags can persist for centuries. But the real world is more complicated than this simple comparison suggests.

Biodegradation Reality: Paper bags only biodegrade properly in the right conditions - they need oxygen, moisture, and microorganisms. In landfills, where most bags end up, these conditions often don't exist. Paper bags can actually persist for years in landfills.

Recycling Rates: Both materials can be recycled, but the rates are different. Paper has higher recycling rates (around 80%) compared to plastic bags (around 12%). However, plastic bags can be recycled multiple times, while paper fibres get shorter each time and can only be recycled 5-7 times.

Reuse Potential: Plastic bags are often reused as bin liners or for storage before disposal. Paper bags are less durable and typically used only once.

5. Conducting Your Own Simple LCA

Let's work through a simplified LCA comparison using real data. This will show you how environmental scientists make these comparisons systematically.

Worked example: Simple LCA Comparison: Plastic vs Paper Shopping Bags

Compare the environmental impact of 1000 plastic bags versus 1000 paper bags across key impact categories.

1. Create categories: Energy (MJ), Water (L), Air pollutants (kg), Solid waste (kg) — These are the main environmental impact categories we'll measure
2. Energy: 763 MJ, Water: 58 L, Air pollutants: 2.7 kg, Solid waste: 7 kg — Data from environmental studies for 1000 plastic bags
3. Energy: 2622 MJ, Water: 1004 L, Air pollutants: 9.1 kg, Solid waste: 33.9 kg — Equivalent data for 1000 paper bags
4. Energy ratio: 2622/763 = 3.43644 Water ratio: 1004/58 = 17.3 Air pollution ratio: 9.1/2.7 = 3.4 Solid waste ratio: 33.9/7 = 4.8 — Paper bags have higher impacts in all manufacturing categories

Answer: Paper bags require 3.4 times more energy, 17.3 times more water, produce 3.4 times more air pollution, and 4.8 times more solid waste during production compared to plastic bags.

This data shows that paper bags have significantly higher environmental impacts during the production phase. However, we must also consider the end-of-life impacts to get the complete picture.

6. Making Evidence-Based Decisions

So which bag should you choose? The LCA reveals that neither option is clearly superior - it depends on what environmental impacts you prioritise and how the bags are actually used and disposed of.

If you prioritise reducing manufacturing impacts: Plastic bags have lower energy use, water consumption, and pollution during production. They're also lighter to transport.

If you prioritise end-of-life impacts: Paper bags biodegrade better under proper conditions and have higher recycling rates.

The best solution: Use reusable bags made from durable materials like cotton or canvas. These have higher initial manufacturing impacts but can be used hundreds of times, making their per-use environmental impact much lower.

This example shows why LCA is so valuable - it prevents us from making decisions based on incomplete information or gut feelings. Environmental problems are complex, and simple solutions often don't work.