It’s summertime and you’re slogging through summer school, so what’s the sunniest spot for all that course reading? Those grassy patches outside Gerstein or behind Robarts, probably.

But there might be more on that lush lawn than you bargained for. There may well be pesticides—chemical poisons intended to kill annoying insects and weeds.

Some of those chemicals have already ended up in your system. Having pesticides in your body doesn’t sound like a good thing, and yet for scientists it’s hard to be sure that it is actually a problem. But that hasn’t stopped Toronto Public Health from wanting to ban pesticides.

Although the amount of chemicals used on public green spaces in Canada is decreasing rapidly, between one-third and one-half of private lawns in Toronto still receive pesticide treatments in the quest for a perfect front yard. But these chemicals don’t stay on the grass. They move through the air, the soil and the water and end up in places they weren’t intended to go.

Toronto’s Public Health department would like to see an end to pesticide use on lawns. It recently prepared a report examining the available research on pesticides and human health. The report cites studies investigating the influence of pesticides on cancers and on the respiratory, nervous, immune, reproductive and hormonal systems.

The effects of these chemicals on human health seem like something we ought to understand. But science doesn’t know much about how they really affect us.

Even more confounding is that a chemical that is harmless in isolation may have serious effects in combination with other agents. It is maddeningly difficult to demonstrate causal links in the complicated real world, especially when both causes and effects may be small, slow and almost undetectable.

The Public Health report says there is no evidence linking pesticides to disease. Nonetheless, the department wants to follow the so-called “precautionary principle,” which simply means that until we know more, we are better safe than sorry. Toronto is now holding public consultations on whether to restrict or even ban the use of pesticides on private land, even though federal government regulators have approved their use.

When chemicals get the Canadian government’s approval, they have to meet certain standards of safety, but those standards are over 30 years old and reforms are moving slowly.

At best, the regulatory standards look at a chemical’s effects in isolation. They don’t require that long-term, low-level exposure be considered or that potential interactions with other environmental chemicals be understood.

Dr. Donald Cole, a member of U of T’s medical faculty in the Department of Public Health Sciences, thinks restrictions are appropriate if we as a community desire long-term sustainability. A ban would be a good choice, says Cole, “if we think that in the face of uncertainty we should take a prudent approach” to the possible risks of using cosmetic pesticides. But “we make decisions based on values,” together with the evidence of science, and not based on science alone. It is possible that another choice of values, Cole admits, like giving priority to maximizing the GDP, would lead us to risk people’s health in favour of economic benefit.

Scientists can do their best to provide some kinds of evidence, but we still need to combine that with the evidence of our own values.

In the precautionary vein, other municipalities have already banned lawn pesticides, and despite legal challenges from manufacturers and lawn maintenance companies, last year the Supreme Court upheld the right of municipal governments to ban “non-essential” pesticides. It’s a striking inversion of the more familiar situation where science and technology bounds ahead of the law (think of stem cell research or the Internet and copyright issues).

In the case of pesticides, political changes may be happening ahead of the science, and if the Public Health department has its way, we in Toronto will be near the front of that wave of change.